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[bot]]

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

```diff @@ 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](https://codecov.io/gh/uber/petastorm/pull/792?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) | Coverage Δ | | |---|---|---| | [petastorm/unischema.py](https://codecov.io/gh/uber/petastorm/pull/792?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber#diff-cGV0YXN0b3JtL3VuaXNjaGVtYS5weQ==) | `93.30% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber)

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