Fix Float field handling of None values

[TVLIgnacy]

Resolves issue #240 by making sure a None value is not attempted to be converted to a float

[codecov]

Codecov Report

Merging #277 (54d7c3a) into master (3614028) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
- Coverage   96.86%   96.83%   -0.04%     
==========================================
  Files          19       19              
  Lines        2712     2714       +2     
==========================================
+ Hits         2627     2628       +1     
- Misses         85       86       +1     

Impacted Files	Coverage Δ
flask_restx/fields.py	96.43% <50.00%> (-0.21%)	⬇️

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[ziirish]

Hello,

Thanks for your contribution.
Could you just add a simple test-case about the issue you are fixing here?

[ziirish]

Looks good to me, but I'd prefer this change to come with a proper test-case.

Thanks

[j5awry]

Merged in #327 which includes the code from this PR + the earlier requested test. Closing this PR.