fido2 list-credentials: Skip enumerating RPs when empty

If we know that the credentials count is zero, we don’t have to
enumerate RPs.  This works around an issue with the Nitrokey FIDO2 not
including the totalRPs field in the enumeration response if it is zero.

Fixes: #336

pynitrokey/cli/fido2.py

@@ -218,8 +218,6 @@ def list_credentials(serial, pin):
         CredentialManagement.RESULT.MAX_REMAINING_COUNT
     )
 
-    reliable_party_list = cred_manager.enumerate_rps()
-
     if cred_count == 0:
         local_print("There are no registered credentials")
         local_print(
@@ -230,6 +228,8 @@ def list_credentials(serial, pin):
     # Get amount of registered creds from first key in list (Same trick is used in the CredentialManager)
     local_print(f"There are {cred_count} registered credentials")
 
+    reliable_party_list = cred_manager.enumerate_rps()
+
     for reliable_party_result in reliable_party_list:
         reliable_party = reliable_party_result.get(CredentialManagement.RESULT.RP)
         reliable_party_hash = reliable_party_result.get(