assert_no_trailing_whitespace fails for all-nan columns

[dweindl]

TypeError: expected string or bytes-like object

To reproduce: run petablint on Raia_CancerResearch2011

@elbaraim : Can you please look into that?

[FFroehlich]

sounds like a python2/3 issue, add/remove b flag when reading file

[dweindl]

No, it's really only a nan on which re.match fails