fix(gapic-generator): Update return types of get_api_endpoint to Optional[str]

Author: hebaalazzeh

packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/_compat.py.j2

@@ -44,9 +44,9 @@ except ImportError:
         universe_domain: str,
         use_mtls_endpoint: str,
         default_universe: str,
-        default_mtls_endpoint: str,
+        default_mtls_endpoint: Optional[str],
         default_endpoint_template: str,
-    ) -> str:
+    ) -> Optional[str]:
         """Return the API endpoint used by the client."""
         if api_override is not None:
             api_endpoint = api_override

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

@@ -364,7 +364,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         client_cert_source: Optional[Callable[[], Tuple[bytes, bytes]]],
         universe_domain: str,
         use_mtls_endpoint: str,
-    ) -> str:
+    ) -> Optional[str]:
         """Return the API endpoint used by the client.
 
         Args:
@@ -376,7 +376,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
                 Possible values are "always", "auto", or "never".
 
         Returns:
-            str: The API endpoint to be used by the client.
+            Optional[str]: The API endpoint to be used by the client.
         """
         return client_utils.get_api_endpoint(
             api_override,