[file_selector] Allow empty type groups

[stuartmorgan]

Description

This was restricted under the assumption that there was no reason for an
empty group to exist, but that's not actually true; on platforms with
selectable groups in the native UI, it may be useful to provide
specific options as well as a fallback option (e.g., "Text files" and
"All files").

Related Issues

None

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Breaking Change

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[ditman]

I'm OK with this change if my suggestion above is not a good way to go!