R1:f5f2a0bc5696

Allow username changes which modify letter case to go through as valid

Summary:
Fixes T13446. Currently, the validation logic here rejects a rename like "alice" to "ALICE" (which changes only letter case) but this is a permissible rename.

Allow collisions that collide with the same user to permit this rename.

Also, fix an issue where an empty rename was treated improperly.

Test Plan:
- Renamed "alice" to "ALICE".
- Before: username collision error.
- After: clean rename.
- Renamed "alice"…