Handle None types

[tclose]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

• Handles None in field types, e.g. int | None instead of typing.Optional[int]

Checklist

• [x] I have added tests to cover my changes (if necessary)
• [x] I have updated documentation (if necessary)

added by @ghisvail Closes #744

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.71%. Comparing base (e52e32b) to head (a7ac191).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #742 +/- ## ========================================== + Coverage 84.27% 91.71% +7.43% ========================================== Files 26 55 +29 Lines 5127 15681 +10554 Branches 1447 2792 +1345 ========================================== + Hits 4321 14382 +10061 - Misses 800 1291 +491 - Partials 6 8 +2 ``` | [Flag](https://app.codecov.io/gh/nipype/pydra/pull/742/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/nipype/pydra/pull/742/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype) | `91.71% <100.00%> (+7.43%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipype#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ghisvail]

I did a superficial read of the diff and what I checked so far looked sensible 👍