Steps to reproduce:

1. Log in to back office.
2. Go to Commerce / Customer groups (v4.3) or Product catalog / Customer groups (v4.4).
3. Add new Customer group with 'Global price rate' set to '-5%'.
4. Edit created Customer group and try to change 'Global price rate' to '0%'.

Actual result:

See attached screenshot

Cannot change global price range to default value which is 0.

Expected result:

User can change Global price rate to default value (0%).