Add `allow_empty` flag to ListField.

[tomchristie]

Possible that it might be useful to have an allow_empty flag that could be set to False to disallow empty selections.
Would be for ListField, ListSerializer, MultipleChoiceField and relations

Eg see. #2804.

[tomchristie]

Reckon this is worth having.

[tomchristie]

Adding a milestone to this to help keep it visible.
May later be dropped or deferred if we don't act on it, but this feature request feels like a good candidate given how widely applicable its use is for relatively little extra complexity.