provider: don't attempt to register unavailable RPs

PR #23380 addressed issue #21785, but with that fix, when an unavailable
resource provider was found, it would still be added to the list of
providers to be registered, and then fail upon attempted registration.

This skips registration for providers which are neither registered nor
unregistered.

Co-Authored-By: Jeff Smith <toxicglados@gmail.com>
Co-Authored-By: Ryan Wozney <wozneyr@users.noreply.github.com>

internal/resourceproviders/requiring_registration.go

@@ -23,6 +23,7 @@ func DetermineWhichRequiredResourceProvidersRequireRegistration(requiredResource
 		if _, isUnregistered := (*unregisteredResourceProviders)[providerName]; !isUnregistered {
 			// some RPs may not exist in some non-public clouds, so we'll log a warning here instead of raising an error
 			log.Printf("[WARN] The required Resource Provider %q wasn't returned from the Azure API", providerName)
+			continue
 		}
 
 		requiringRegistration = append(requiringRegistration, providerName)