Fix spurious errors when running validator with --skip-optional

[ml-evs]

Closes #1558

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0060f44). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1559   +/-   ##
=========================================
  Coverage          ?   91.09%           
=========================================
  Files             ?       74           
  Lines             ?     4538           
  Branches          ?        0           
=========================================
  Hits              ?     4134           
  Misses            ?      404           
  Partials          ?        0           

Flag	Coverage Δ
project	91.09% <100.00%> (?)
validator	90.98% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/validator/utils.py	94.59% <100.00%> (ø)
optimade/validator/validator.py	83.69% <100.00%> (ø)

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[ml-evs]

Thanks @JPBergsma, well caught! I'll merge this now.