Avoid NPE if commit is called before any offset is prepared

2018-07-25 15:12:40,386 ERROR  OracleConnectorIT||engine  Stopping connector after error in the application's handler method: null   [io.debezium.connector.oracle.OracleConnectorIT]
java.lang.NullPointerException
    at io.debezium.connector.oracle.OracleStreamingChangeEventSource.commitOffset(OracleStreamingChangeEventSource.java:93)
    at io.debezium.pipeline.ChangeEventSourceCoordinator.commitOffset(ChangeEventSourceCoordinator.java:83)
    at io.debezium.connector.oracle.OracleConnectorTask.commit(OracleConnectorTask.java:123)
    at io.debezium.embedded.EmbeddedEngine.commitOffsets(EmbeddedEngine.java:833)
    at io.debezium.embedded.EmbeddedEngine.maybeFlush(EmbeddedEngine.java:811)
    at io.debezium.embedded.EmbeddedEngine.run(EmbeddedEngine.java:733)
    at io.debezium.embedded.AbstractConnectorTest.lambda$start$3(AbstractConnectorTest.java:297)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)