SCSS hardening

[juliusknorr]

• Do not assume that empty CSS files are cached
• Use default scss variables in case the injected variables are somehow invalid (fixesCSS is not properly rendered after upgrade from 11 to 12.0.4 #7977)
• Make sure we only build the injected variables string once
• Do not rewrite absolute URLs (starting with /)
  • With this we can remove the getAbsoluteUrl call from the the theming app, so the SCSSCacher will not interpret different domains as changes (fixesNC12 slow with multiple domains #6752)

[codecov]

Codecov Report

Merging #8078 into master will decrease coverage by 45.09%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #8078 +/- ##
===========================================
- Coverage 51.7% 6.6% -45.1% - Complexity 25426 25428 +2 
===========================================
Files 1598 1598 Lines 95241 95251 +10 Branches 1376 1376 ===========================================
- Hits 49243 6293 -42950 - Misses 45998 88958 +42960

Impacted Files	Coverage Δ	Complexity Δ
apps/theming/lib/ThemingDefaults.php	0% <0%> (-91.8%)	46 <0> (ø)
lib/private/Template/SCSSCacher.php	0% <0%> (-71.1%)	39 <0> (+2)
...eLimiting/Exception/RateLimitExceededException.php	0% <0%> (-100%)	1% <0%> (ø)
...yBuilder/FunctionBuilder/SqliteFunctionBuilder.php	0% <0%> (-100%)	1% <0%> (ø)
lib/private/Remote/InstanceFactory.php	0% <0%> (-100%)	2% <0%> (ø)
apps/files_versions/lib/AppInfo/Application.php	0% <0%> (-100%)	2% <0%> (ø)
apps/encryption/lib/Session.php	0% <0%> (-100%)	21% <0%> (ø)
apps/oauth2/lib/Db/Client.php	0% <0%> (-100%)	1% <0%> (ø)
apps/dav/lib/Connector/Sabre/Server.php	0% <0%> (-100%)	1% <0%> (ø)
...b/public/AppFramework/Db/DoesNotExistException.php	0% <0%> (-100%)	1% <0%> (ø)
... and 860 more

[rullzer]

yes please... the old code was clearly me messing up 😉

[skjnldsv]

Awesome!!! Thanks @juliushaertl !

[rullzer]

🚀

[juliusknorr]

backport at #8156