Automatically drop empty facets on pairwise comparisons figure

[nebfield]

I hardcoded facets for T2D but realised some endpoints have limited data, like Urate

Also the stroke endpoint looks very weird. Should I add some endpoint exclusions @remomomo ?

https://intervene-eu-h2020.github.io/prs-method-compare/pairwise.html

[nebfield]

21ae99f