Crash when reloading more than one backuped empty project

[kleonc]

Pixelorama version: 4001a32f2b2badc3cd31cd49e09ebbf4d8378473

Issue description: Title.
Seems like OpenSave's member arrays: current_save_paths and backup_save_paths are assumed to be the same size as arguments passed to the OpenSave.reload_backup_file method which seems to not always be the case.

Steps to reproduce:

1. Create at least 2 empty projects (with nothing being drawn) and wait for each of them be autosaved.
2. Terminate the program (for example by clicking stop button in the Godot). Closing Pixelorama with the X button seems to not cause the crash.
3. Run Pixelorama again and confirm reloading backups.

[OverloadedOrama]

Seems like the issue happens because importing a new project when the current one is empty, replaces the empty project instead of creating a new tab. This behavior happens when importing backups too, the first project is empty, so the second one replaces it, thus a new value is not being appended in backup_save_paths[], which results in the error. It should get fixed in the next commit, so empty backups will not get replaced.