Fix SVG rendering error in SvgImageProvider

[claucambra]

As part of IconUtils::createSvgImageWithCustomColor we try to see if there is a pre-existing image with the desired colour combination string at a given path. However, we do not check if the file exists and instead try directly to create the image; this creates an unnecessary error when we could just check for the existence of the file at the path.

2023-05-09 14:49:16:215 [ warning qt.svg unknown:0 ]: Cannot open file ':/client/theme/black/share.svg', because: No such file or directory

This PR adds this file check first, avoiding the error.

Additionally, this PR includes some clean up of the relevant code.

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

88.5% Coverage
0.0% Duplication

[codecov]

Codecov Report

Merging #5666 (7edfe21) into master (759c2a2) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 7edfe21 differs from pull request most recent head ed05436. Consider uploading reports for the commit ed05436 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5666      +/-   ##
==========================================
- Coverage   60.08%   60.06%   -0.03%     
==========================================
  Files         145      145              
  Lines       18756    18746      -10     
==========================================
- Hits        11270    11260      -10     
  Misses       7486     7486              

see 5 files with indirect coverage changes

[claucambra]

/backport to stable-3.9

[allexzander]

@claucambra Please squash before merging

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
3 Code Smells

76.0% Coverage
0.0% Duplication

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[nextcloud-desktop-bot]

AppImage file: nextcloud-PR-5666-ed05436ea572a4e17f0d43d6913a345c60b19935-x86_64.AppImage

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