[PDI-9088] Junit test added

Prioritize streams step causing null pointer exception downstream.
new file:
engine/test-src/org/pentaho/di/trans/steps/prioritizestreams/PrioritizeStreamsTest.java
[PDI-9088] fix added

Prioritize streams step causing null pointer exception downstream. Fix
added based on Frank Hilgendorf proposal in attachment to the bug, thank
you very much.
modified:
engine/src/org/pentaho/di/trans/steps/prioritizestreams/PrioritizeStreams.java
Merge pull request #184 from prakapenka/PDI-9088

PDI-9088 - Prioritize streams step causing null pointer exception downstream