This PR was created by Knope. Merging it will create a new release

### Fixes

#### Remove spurious field_dict.update({}) for types without properties
(#969)

#### Fix invalid type check for nested unions

Nested union types (unions of unions) were generating `isinstance()`
checks that were not valid (at least for Python 3.9).

Thanks to @codebutler for PR #959 which fixes #958 and #967.

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