Add missing field_name in ValueError

[chasleslr]

I encountered this error a few times and noticed the variable name was missing. Without this commit, the error currently reads:

Field {} is not "nullable", but got passes a None value

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[selitvin]

Thanks for the fix! So we can land your change, can you please accept the CLA?

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0b0775a) 86.24% compared to head (b808fc1) 86.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #792   +/-   ##
=======================================
  Coverage   86.24%   86.24%           
=======================================
  Files          84       84           
  Lines        5090     5090           
  Branches      790      790           
=======================================
  Hits         4390     4390           
  Misses        560      560           
  Partials      140      140           

Impacted Files	Coverage Δ
petastorm/unischema.py	93.30% <100.00%> (ø)

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