Null pointers should not be dereferenced (controller)

Following code points lack null safety:

• controller/src/main/java/org/jboss/as/controller/OperationContextImpl.java #1031
• controller/src/main/java/org/jboss/as/controller/operations/global/ReadResourceDescriptionHandler.java #828
• controller/src/main/java/org/jboss/as/controller/operations/validation/OperationValidator.java #13
• controller/src/main/java/org/jboss/as/controller/persistence/yaml/YamlConfigurationExtension.java #501
• controller/src/main/java/org/jboss/as/controller/registry/OperationTransformerRegistry.java #358