fix: validate name when renaming in editor

[ZebraVogel94349]

I've noticed that validation of the new name when renaming a note in the editor doesn't work, so you can rename a note to an empty name or a name containing slashes from the editor. The problem was that _filenameFormKey.currentState?.validate() ?? true is always a string or true, but it should be false if the function returns null. I've also fixed a bug which made it impossible to save a note after renaming it in the editor but before the new name has been applied.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 36.97%. Comparing base (f3afcea) to head (a2a29c5).

Files	Patch %	Lines
lib/pages/editor/editor.dart	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1213      +/-   ##
==========================================
+ Coverage   36.93%   36.97%   +0.04%     
==========================================
  Files         111      111              
  Lines        8715     8716       +1     
==========================================
+ Hits         3219     3223       +4     
+ Misses       5496     5493       -3     

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