fix: invalid scoped-sync responses for empty flags

[toddbaert]

Fixes an issue where invalid flag payloads were returned on sync requests with scopes if the flag set was empty.

Below is an example of the bug.

$ grpcurl -import-path /home/todd/temp -proto sync.proto -plaintext localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags
{
  "flagConfiguration": "{\"flags\":{}}"
}
$ grpcurl -import-path /home/todd/temp -proto sync.proto -plaintext -d '{"selector":"../config/samples/example_flags.flagd.json"}' localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags 
{}

[netlify[bot]]

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