TVPaint: Fix renaming of rendered files

[iLLiCiTiT]

Brief description

Change return value of 'rename_filepaths_by_frame_start' when frame start is same as mark in.

Testing notes:

1. Set frame start of an asset to 0
2. Start tvpaint on the asset's task
3. Create some render layers to render
4. Run publish
5. Publishing should not crash during extraction

[ynbot]

Task linked: OP-4100 TVPaint same render frame crashes