Always trigger a build if there's no previous one

[jorgenpt]

When we can't determine the previously built change, always trigger a build. Otherwise, Perforce triggers and Perforce polling will never trigger a build until a manual build has been run.

It also seems that if a build fails before recording a last synced change, it will "revert" to this "unbuilt" state, so further triggers will not build the job.

This also seems to have been the intended functionality, if you look in PerforceScm.java on line 495 it says Calculate last change, build if null

[jorgenpt]

Seems like a legitimate test failure, I'm investigating.

[skumar7322]

Hi @jorgenpt thank you for creating this PR. The use case of triggering build if there are no changes is not ever one wanted. If the last build is successful and no change is submitted to perforce then it is desired to not trigger a build.