External links from decision table to other XLS files are not handled correctly

Description of problem:
Decision table is not correctly compiled when using links to external XLS files. See attached reproducer.

I think the external link should not result in error even if it is not available (broken) -> the previous known (cached) value should be used.

This is the regression from 5.3.1 where both available and not-available (broken) links worked.

Full stacktrace:
java.lang.RuntimeException: Could not resolve external workbook name 'home/psiroky/work/workspace/decisiontables-example-drools6/src/test/resources/definitions.xls'. Workbook environment has not been set up.
at org.apache.poi.ss.formula.OperationEvaluationContext.createExternSheetRefEvaluator(OperationEvaluationContext.java:89)
at org.apache.poi.ss.formula.OperationEvaluationContext.getRef3DEval(OperationEvaluationContext.java:249)
at org.apache.poi.ss.formula.WorkbookEvaluator.getEvalForPtg(WorkbookEvaluator.java:656)
at org.apache.poi.ss.formula.WorkbookEvaluator.evaluateFormula(WorkbookEvaluator.java:527)
at org.apache.poi.ss.formula.WorkbookEvaluator.evaluateAny(WorkbookEvaluator.java:288)
at org.apache.poi.ss.formula.WorkbookEvaluator.evaluate(WorkbookEvaluator.java:230)
at org.apache.poi.hssf.usermodel.HSSFFormulaEvaluator.evaluateFormulaCellValue(HSSFFormulaEvaluator.java:354)
at org.apache.poi.hssf.usermodel.HSSFFormulaEvaluator.evaluate(HSSFFormulaEvaluator.java:185)
at org.drools.decisiontable.parser.xls.ExcelParser.processSheet(ExcelParser.java:145)
at org.drools.decisiontable.parser.xls.ExcelParser.parseFile(ExcelParser.java:79)
at org.drools.decisiontable.SpreadsheetCompiler.compile(SpreadsheetCompiler.java:93)
at org.drools.decisiontable.SpreadsheetCompiler.compile(SpreadsheetCompiler.java:72)
at org.drools.decisiontable.DecisionTableProviderImpl.compileStream(DecisionTableProviderImpl.java:37)
at org.drools.decisiontable.DecisionTableProviderImpl.loadFromInputStream(DecisionTableProviderImpl.java:20)
at org.drools.compiler.compiler.DecisionTableFactory.loadFromInputStream(DecisionTableFactory.java:15)
at org.drools.compiler.compiler.PackageBuilder.decisionTableToPackageDescr(PackageBuilder.java:461)
at org.drools.compiler.compiler.CompositeKnowledgeBuilderImpl.buildPackageDescr(CompositeKnowledgeBuilderImpl.java:348)
at org.drools.compiler.compiler.CompositeKnowledgeBuilderImpl.buildPackages(CompositeKnowledgeBuilderImpl.java:84)
at org.drools.compiler.compiler.CompositeKnowledgeBuilderImpl.build(CompositeKnowledgeBuilderImpl.java:75)
at org.drools.compiler.kie.builder.impl.AbstractKieModule.buildKnowledgePackages(AbstractKieModule.java:140)
at org.drools.compiler.kie.builder.impl.AbstractKieProject.verify(AbstractKieProject.java:39)
at org.drools.compiler.kie.builder.impl.KieBuilderImpl.buildKieModule(KieBuilderImpl.java:207)
at org.drools.compiler.kie.builder.impl.KieBuilderImpl.buildAll(KieBuilderImpl.java:160)
at org.kie.internal.utils.KieHelper.build(KieHelper.java:25)
at org.jboss.qa.brms.dt.example.CreateKBaseFromDTTest.createKBase(CreateKBaseFromDTTest.java:18)
at org.jboss.qa.brms.dt.example.CreateKBaseFromDTTest.testCreateKBasefromDT(CreateKBaseFromDTTest.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: org.apache.poi.ss.formula.CollaboratingWorkbooksEnvironment$WorkbookNotFoundException: Could not resolve external workbook name 'home/psiroky/work/workspace/decisiontables-example-drools6/src/test/resources/definitions.xls'. Workbook environment has not been set up.
at org.apache.poi.ss.formula.CollaboratingWorkbooksEnvironment.getWorkbookEvaluator(CollaboratingWorkbooksEnvironment.java:161)
at org.apache.poi.ss.formula.WorkbookEvaluator.getOtherWorkbookEvaluator(WorkbookEvaluator.java:181)
at org.apache.poi.ss.formula.OperationEvaluationContext.createExternSheetRefEvaluator(OperationEvaluationContext.java:87)
... 48 more

Version-Release number of selected component (if applicable):
6.0.0-SNAPSHOT(2013-05-27)

How reproducible:
Always

Steps to Reproduce:
1. Update the link (cell C7) to definitions.xls in sample.xls.
2. Run the attached reproducer (mvn test)

Actual results:
Exception when compiling the XLS file.

Expected results:
Successful compilation.