cubicweb #1624839 [ui] tristate boolean widget shows dummy entry [validation pending]

see attachment

this happens when using BooleanField(allow_none=True)

prioritynormal
typebug
done in3.12.3
load left0.000
closed by#18bcb98d129a [widgets] handle no value label for tristate radios, put default separator on class for easier redefinition - closes #1624839