In almost all cases there is no difference between `G.relbase_valid`
and checking `G.main->filepath` isn't an empty string.
In many places a non-empty string is already being used instead of
`G.relbase_valid`.
The only situation where this was needed was when saving from
`wm_file_write` where they temporarily became out of sync.
This has been replaced by adding a new member to `BlendFileWriteParams`
to account for saving an unsaved file for the first time.
Reviewed By: brecht
Ref D13564
5de109cc2d