Fix edge case around NaN values (#4964)

[john-bodley]

(cherry picked from commit e1618dd)

This PR fixes an issue reported in the #superset-goalie channel, Unable to parse string "NaN" at position 0.

to: @graceguo-supercat @kristw @michellethomas @timifasubaa

[codecov-io]

Codecov Report

Merging #92 into airbnb-master will not change coverage.
The diff coverage is 0%.

@@              Coverage Diff               @@
##           airbnb-master      #92   +/-   ##
==============================================
  Coverage          63.25%   63.25%           
==============================================
  Files                351      351           
  Lines              22034    22034           
  Branches            2468     2468           
==============================================
  Hits               13937    13937           
  Misses              8085     8085           
  Partials              12       12

Impacted Files	Coverage Δ
superset/viz.py	80.89% <0%> (ø)	⬆️

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