docval with string enum should allow default value of None

[rly]

Description

The following docval argument raises an error:

{'name': 'continuity', 'type': str, 'default': None, 'enum': ["continuous", "instantaneous", "step"],
'doc': 'Optionally describe the continuity of the data.'}

Docval should allow this use case of an optional field where, when provided, the value must be an item from the list. Alternatively, the enum list should include None, but that seems ugly.

Environment

Python Executable: Python
Python Version: Python 3.8
Operating System: Windows
HDMF Version: dev

Checklist

• Have you ensured the feature or change was not already reported ?
• Have you included a brief and descriptive title?
• Have you included a clear description of the problem you are trying to solve?
• Have you included a minimal code snippet that reproduces the issue you are encountering?
• Have you checked our Contributing document?

[rly]

This is blocking NeurodataWithoutBorders/pynwb#1226