Cache final result of update check

[MorrisJobke]

If the parsed data is not a valid response we should not cache it and only cache the preprocessed result set.

Fixes#7442

This method is called in

server/apps/updatenotification/lib/UpdateChecker.php

Line 44 in 0eebff1

$data = $this->updater->check();

If the $data instead of $tmp is cached, then the returned value in the first call and the second call could be different, because a different object is cached causing the array access error in #7442.

I also added a unit test to verify that a missing attribute still results in a proper result with all keys.

[rullzer]

Makes sense

[codecov]

Codecov Report

Merging #7562 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #7562 +/- ##
============================================
- Coverage 51.18% 51.17% -0.01% 
Complexity 24874 24874 ============================================
Files 1601 1601 Lines 94701 94700 -1 Branches 1368 1368 ============================================
- Hits 48474 48467 -7 - Misses 46227 46233 +6

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Updater/VersionCheck.php	92.1% <100%> (-0.21%)	7 <0> (ø)
core/js/js.js	62.98% <0%> (-0.57%)	0% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.28% <0%> (-0.25%)	136% <0%> (ø)