Check isnan function for empty strings

Bioinformatics Add-on often uses isnan to check for unknowns even in places, where one would expect string values. isnan fails for strings. Check whether these functions work and how. Fix the issue for empty strings accordingly.

Widgets that use isnan:
- OWFeatureSelection
- OWGeneInfo
- OWEnrichmentAnalysis
- OWKEGG
- OWSelectGenes

Locate the isnan calls in OWFeatureSelection, OWGeneInfo, OWEnrichmentAnalysis, OWKEGG, and OWSelectGenes, then reproduce their behavior with empty strings. Determine which functions fail and verify that all listed widgets handle empty-string values correctly.

Written by the indexing model from the issue text.