Fix handling of Spearman correlation (et al) with NaN value from server

[alisman]

Related to cBioPortal/cbioportal#9148

Issue comes from this service when you submit only one bin.

https://www.cbioportal.org/api/clinical-data-density-plot/fetch?xAxisAttributeId=FRACTION_GENOME_ALTERED&xAxisBinCount=44&xAxisEnd=1&xAxisLogScale=false&xAxisStart=0&yAxisAttributeId=MUTATION_COUNT&yAxisBinCount=38&yAxisLogScale=false&yAxisStart=0

Service returns NaN for the correlation values which javascript then barfs on. This PR will just print NaN instead of white screen so it's improvement.

There seems to be a problem with selection mechanism wherein if you choose less than four bars, it considers it just a single bar. Questions are:
A) should service return NaN?
B) should selection UI allow selection of a single bar? When you drag from within a bar, how should that affect selection?

[inodb]

Thanks!

[inodb]

@alisman My thoughts on the questions:

A) I think it makes sense to return NA when no correlation can be computed with given input
B) Selection of single bar makes sense to me. When starting from within a bar, it should select the whole bar