Fix categories parsing, when eTag 304 is used

[BPerlakiH]

Fixes: #1007

As described in the re-opened issue, we need to handle the memory only stored categories to languages, when there's no new data coming from the http fetch (since it's 304, thanks to eTag).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.

Project coverage is 36.87%. Comparing base (2a5195d) to head (cfdce82).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ViewModel/LibraryViewModel.swift	0.00%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1063      +/-   ##
==========================================
- Coverage   37.21%   36.87%   -0.34%     
==========================================
  Files         127      127              
  Lines        7519     7492      -27     
==========================================
- Hits         2798     2763      -35     
- Misses       4721     4729       +8     

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