Add optional test for whether entry endpoints have any actual entries

[ml-evs]

Closes #1890

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (eaa4315) 90.86% compared to head (68e5973) 90.81%.

Files	Patch %	Lines
optimade/validator/validator.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1891      +/-   ##
==========================================
- Coverage   90.86%   90.81%   -0.05%     
==========================================
  Files          75       75              
  Lines        4652     4661       +9     
==========================================
+ Hits         4227     4233       +6     
- Misses        425      428       +3     

Flag	Coverage Δ
project	90.81% <66.66%> (-0.05%)	⬇️
validator	90.68% <66.66%> (-0.05%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.