Make NullBooleanSelect.format_value() return a list, consistent with ChoiceWidget

[Brian Helba]

ChoiceWidget.format_value() is documented (in its docstring) to "Return selected values as a list", and every ChoiceWidget subclass honors that, except NullBooleanSelect, which returns a bare string: "unknown", "true", or "false".

NullBooleanSelect does not override get_context() or optgroups(). It relies on the inherited ChoiceWidget.optgroups(), which marks an option as selected with str(subvalue) in value. When value is a list that is list membership. When value is the bare string it becomes a substring test, which only produces correct results because none of "unknown", "true", or "false" happens to be a substring of another. The parent's contract is satisfied by accident rather than by design, and it may be confusing for anyone who later customizes the choices or the parent implementation.

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History: #27866 (1.11) changed ChoiceWidget.format_value() from returning a set to returning a list but did not touch NullBooleanSelect. #17210 (2.2) changed the NullBooleanSelect mapping from 1/2/3 to unknown/true/false and kept returning a bare string.

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Proposed change: wrap the result in a one-element list, i.e. ["unknown"], ["true"], or ["false"]. The rendered HTML should be identical. The only impact would be for third-party code that calls widget.format_value() directly and uses the result as a value representation.

[Brian Helba]

​https://github.com/django/django/pull/22009