Find methods could return null

[core23]

Subject

Call a findGroupBy* could find no group and could return null. Technically an API BC break, but this should have never worked.

[codecov]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #38   +/-   ##
=========================================
  Coverage     54.34%   54.34%           
- Complexity      480      483    +3     
=========================================
  Files            62       62           
  Lines          1301     1301           
=========================================
  Hits            707      707           
  Misses          594      594

Impacted Files	Coverage Δ	Complexity Δ
src/Model/GroupManager.php	0% <0%> (ø)	3 <2> (+1)	⬆️
src/Noop/GroupManager.php	0% <0%> (ø)	6 <2> (+1)	⬆️
src/Doctrine/GroupManager.php	0% <0%> (ø)	8 <2> (+1)	⬆️

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