Correctly use masked arrays after unstructured_nearest regridding

[schlunma]

Description

Right now, unstructured_nearest regridding does not return masked arrays. This PR fixes this.

Closes #1413

Link to documentation:

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[codecov]

Codecov Report

Merging #1414 (9c283ed) into main (5131ffc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1414   +/-   ##
=======================================
  Coverage   88.85%   88.86%           
=======================================
  Files         196      196           
  Lines        9959     9965    +6     
=======================================
+ Hits         8849     8855    +6     
  Misses       1110     1110           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_regrid.py	96.73% <100.00%> (+0.07%)	⬆️

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[valeriupredoi]

looks good! when iris will have fixed this it'd be good to remove the condition but leave the test - I'd argue an issue about this on GH scitools-iris would be good to have 👍

[schlunma]

I found an issue with this, please do not merge yet 😬

[valeriupredoi]

you merge at will, man, am off on baby Jesus holidays 🎄 🎅

[schlunma]

Cheers, happy holidays!! 🎅 🎄