Improve preprocessor output sorting code

[bouweandela]

Description

Follow up on #2103. This reduces duplicate code and avoids a potential crash if for some reason recipe_dataset_index would not be set (which seems unlikely).

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[codecov]

Codecov Report

Merging #2111 (6c6a839) into main (26b50fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2111   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files         237      237           
  Lines       12804    12806    +2     
=======================================
+ Hits        11920    11922    +2     
  Misses        884      884           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/__init__.py	95.23% <100.00%> (ø)
esmvalcore/preprocessor/_io.py	86.85% <100.00%> (+0.10%)	⬆️

[valeriupredoi]

nice and tidy 🍺