Change validation error to warning on empty endpoints

[ml-evs]

Closes #622.

[codecov]

Codecov Report

Merging #621 (b8a08ab) into master (1ffb03d) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #621   +/-   ##
=======================================
  Coverage   92.20%   92.20%           
=======================================
  Files          61       61           
  Lines        3246     3246           
=======================================
  Hits         2993     2993           
  Misses        253      253           

Flag	Coverage Δ
project	92.20% <0.00%> (ø)
validator	65.89% <0.00%> (ø)

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Impacted Files	Coverage Δ
optimade/validator/validator.py	82.26% <0.00%> (ø)

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[ml-evs]

Self-merging, as this is a tiny but useful change.