fix(generator): update fallback signatures and type cast client_cert_source

Author: hebaalazzeh

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

@@ -62,16 +62,15 @@ except ImportError:
             return default_endpoint_template.format(UNIVERSE_DOMAIN=universe_domain)
 
     def get_universe_domain(
-        client_universe_domain: Optional[str],
-        universe_domain_env: Optional[str],
+        *potential_universes: Optional[str],
         default_universe: str,
     ) -> str:
         """Return the universe domain used by the client."""
         universe_domain = default_universe
-        if client_universe_domain is not None:
-            universe_domain = client_universe_domain
-        elif universe_domain_env is not None:
-            universe_domain = universe_domain_env
+        for potential_universe in potential_universes:
+            if potential_universe is not None:
+                universe_domain = potential_universe
+                break
         if len(universe_domain.strip()) == 0:
             raise ValueError("Universe Domain cannot be an empty string.") 
         return universe_domain

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

@@ -379,7 +379,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
             str: The API endpoint to be used by the client.
         """
         use_mtls = use_mtls_endpoint == "always" or (
-            use_mtls_endpoint == "auto" and client_cert_source
+            use_mtls_endpoint == "auto" and bool(client_cert_source)
         )
         return client_utils.get_api_endpoint(
             api_override,