[flagd-provider] add unit test for zero-value resolutions from flagd

[toddbaert]

Flagd sends empty values in flag response payloads when the value is equivalent to the zero-value of the associated go type. We should test that this works as expected and still returns an equivalent zero-value (ie: false, 0).

DoD:

• manual test confirming zero values are returned from the flagd provider when they are missing from the gRPC message (boolean set to false or numeric set to 0 in flagd config)
• new tests that assert correct values for zero-value flags

see: open-feature/flagd#218

[odubajDT]

Hi, I would like to work on this ticket :)