UnprocessedCommitWalker: check for ZERO_ID before parsing

For a created branch commitSha1TransactionEnd is set to ZERO_ID, but
we check that first after we are trying to parse RevCommit from it.
Set transactionEnd to null if commitSha1TransactionEnd is equal to
ZERO_ID.

Solves: Jira GER-1611
Change-Id: I921752bd8140c0e8ff6e2a5829baac3e3554ccfc