SP-1433 - Backport of PDI-12318 - NullPointerException running job with massive parallel Job/Transformation loads (5.1)

backport on 5.1 branch.

	modified:   engine/src/org/pentaho/di/job/Job.java
Merge pull request #697 from prakapenka/SP-1433

SP-1433 - Backport of PDI-12318 - NullPointerException running job with massive parallel Job/Transformation loads (5.1)