[Pimcore UI] Default Product Unit always persisted, cannot unselect

[aarongerig]

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	yes

If at least one product unit is defined and you open the tab «Price & Units» of a Product object in the Pimcore UI, it is impossible to save a it without the default product unit definition.

See the behavior in the attached video.

https://user-images.githubusercontent.com/17384333/178445389-12825ad1-5204-4330-a55e-9467dfa833c0.mov