Fix a few bugs where Flow was not working on pristine pages.

* Flow Edit Header API module required a prev_revision parameter, when none is passed for the first edit.
* Topic List History was throwing an exception when there was no history to find.
* Flow Header block thought that no prev_revision parameter would be passed at all for the first rev,
  whereas the API now passes an empty string if no value is given

Bug: 63910
Change-Id: I3373e58c9e0a8ab96f16bd306c43ea12cb272bcc
Merge "Fix a few bugs where Flow was not working on pristine pages."