Support "allow_none=True" in docval for args with non-None default

[rly]

Motivation

Addressing issue raised in NeurodataWithoutBorders/pynwb#1540 (comment)

How to test the behavior?

@docval({'name': 'arg1', 'type': bool, 'doc': 'this is a bool or None with a default', 'default': True,
                 'allow_none': True})
    def method(self, **kwargs):
        return popargs('arg1', kwargs)

    # if provided, None is allowed
    res = method(self, arg1=None)
    self.assertIsNone(res)

    # if not provided, the default value is used
    res = method(self)
    self.assertTrue(res)

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #757 (7246b49) into dev (94e9e3d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #757   +/-   ##
=======================================
  Coverage   87.59%   87.59%           
=======================================
  Files          44       44           
  Lines        8834     8837    +3     
  Branches     2034     2035    +1     
=======================================
+ Hits         7738     7741    +3     
  Misses        781      781           
  Partials      315      315           

Impacted Files	Coverage Δ
src/hdmf/utils.py	96.41% <100.00%> (+0.01%)	⬆️

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[rly]

Test failures are upstream in the use of conda in github actions.