set api_endpoint to empty string

Author: daniel-sanche

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -567,7 +567,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         self._use_client_cert, self._use_mtls_endpoint, self._universe_domain_env = {{ service.client_name }}._read_environment_variables()
         self._client_cert_source = {{ service.client_name }}._get_client_cert_source(self._client_options.client_cert_source, self._use_client_cert)
         self._universe_domain = {{ service.client_name }}._get_universe_domain(universe_domain_opt, self._universe_domain_env)
-        self._api_endpoint: str  # updated below, depending on `transport`
+        self._api_endpoint: str = ""  # updated below, depending on `transport`
 
         # Initialize the universe domain validation.
         self._is_universe_domain_valid = False