Directly fix invalid values of DTEND and DTSTART

[nickvergessen]

Ref #4014

Fix#2741
Fix#3350
Fix#3734

@georgehrke@rullzer

[mention-bot]

@nickvergessen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rullzer to be a potential reviewer.

[codecov-io]

Codecov Report

Merging #4066 into master will decrease coverage by 0.01%.
The diff coverage is 59.18%.

@@ Coverage Diff @@## master #4066 +/- ##
============================================
- Coverage 54.24% 54.22% -0.02% + Complexity 21285 21283 -2 
============================================
Files 1310 1309 -1 Lines 81187 81198 +11 Branches 1284 1284 ============================================
- Hits 44036 44030 -6 - Misses 37151 37168 +17

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/Migration/CalDAVRemoveEmptyValue.php	59.18% <59.18%> (ø)	9 <9> (?)
apps/comments/lib/EventHandler.php	79.16% <0%> (-8.34%)	7% <0%> (ø)
apps/files_external/lib/Lib/Storage/SMB.php	47.22% <0%> (+0.39%)	112% <0%> (ø)	⬇️

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[MorrisJobke]

@nickvergessen So this is intended to be run on every upgrade? Otherwise add a version number check.

[nickvergessen]

Basically yes. It only does stuff for broken entries anyway

[MorrisJobke]

Looks good 👍

[rullzer]

Looks good to me. And fixed some broken stuff I threw at it.

So lets do it...