Fix manual verification of elements_ratios

[CasperWA]

I had some issues regarding validation of elements_ratios and found out that an equal sign was all that was missing for the comparison against EPS.

Also, add the valid None or null as response with single datum (StructureResponseOne), if it cannot be found, in accordance with the OPTiMaDe spec.

[codecov]

Codecov Report

Merging #76 into master will decrease coverage by 0.05%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   85.71%   85.66%   -0.06%     
==========================================
  Files          30       30              
  Lines        1323     1325       +2     
==========================================
+ Hits         1134     1135       +1     
- Misses        189      190       +1

Impacted Files	Coverage Δ
optimade/models/structures.py	87.25% <ø> (ø)	⬆️
optimade/models/toplevel.py	100% <100%> (ø)	⬆️
optimade/server/entry_collections.py	81.96% <75%> (-0.54%)	⬇️

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[ml-evs]

LGTM!

Can we make one extra change?

Now that the single entry endpoint can return None when there are no results, can we change the linked line below to if data_available > 1? This suppresses an error on my server when querying a non-existent structure.

https://github.com/CasperWA/optimade-python-tools/blob/9f2689ff3d67b0e8407c24b0cd3e49ae4116bde8/optimade/server/entry_collections.py#L109

[ml-evs]

LGTM!

Can we make one extra change?

Now that the single entry endpoint can return None when there are no results, can we change the linked line below to if data_available > 1? This suppresses an error on my server when querying a non-existent structure.

https://github.com/CasperWA/optimade-python-tools/blob/9f2689ff3d67b0e8407c24b0cd3e49ae4116bde8/optimade/server/entry_collections.py#L109

Couple of extra lines needed changing too so I did it myself.

[ml-evs]

Check my changes and merge when you're happy

[CasperWA]

Thanks @ml-evs - and yeah, I have made several of those changes in my own server as well, simply forgot to add them here.
Please review and accept if you are all for it, then I'll merge :)