NullStageHistoryItem does not have identifier & causes NPE on .getPipelineName(). adding check.
fixing feedback:
* comment on PHS.loadMinimalData()
* removed modified files from pipeline history & material commits API
* removed currently_locked, lockable & can_unlock from pipeline history API since it was not populated
Merge pull request #623 from srinivasupadhya/api-fixes

Api fixes