MODE-1978 Corrected the WritableSessionCache to fix NPE from recently-added code 

A recent fix for MODE-1940 added code that updated the indexes if the new parent was persisted (previously) and the child was moved. In some cases when the new parent was not persisted in a previous save but instead was created as part of this save, the code resulted in an NPE. The fix was a simple change.

A test case was added to first replicate the problem. After the change, the test now succeeds (and the S-RAMP build works completely).

Ready for review and merging into 'master' and '3.3.x'.