GROOVY-11308: Restore compatibility of Collection#unique

[octylFractal]

The documentation for this method states that when mutate is false, a new collection is always returned. The previous code only did so when self.size() was greater than 1, and otherwise returned null.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.5311%. Comparing base (4725c43) to head (e17ebc6).
Report is 485 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2056        +/-   ##
==================================================
+ Coverage     68.5281%   68.5311%   +0.0030%     
+ Complexity      29177      29176         -1     
==================================================
  Files            1422       1422                
  Lines          113495     113493         -2     
  Branches        19548      19547         -1     
==================================================
+ Hits            77776      77778         +2     
+ Misses          29178      29175         -3     
+ Partials         6541       6540         -1     

Files with missing lines	Coverage Δ
.../codehaus/groovy/runtime/DefaultGroovyMethods.java	73.6213% <100.0000%> (+0.0157%)	⬆️

... and 2 files with indirect coverage changes

[paulk-asert]

Merged, thanks! There is a small tweak we can do to get the minor performance gain that I think the original code was after, but I can do that after the merge.