Displaying NaN scores in heatmap

[afrubin]

For most datasets, variants that don't have scores are simply not present and are visualized in the heatmap as white squares.

However, there are other datasets like https://www.mavedb.org/score-sets/urn:mavedb:00000050-a-1 where some variants explicitly have a score of NaN. Right now, those show up as (extremely prominent) black squares, but they should probably have white squares like regular missing data.