handle none case in _get_default_mtls_endpoint

Author: daniel-sanche

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/client.py.j2

@@ -118,6 +118,9 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         )
 
         m = mtls_endpoint_re.match(api_endpoint)
+        if m is None:
+            # could not parse api_endpoint; return as-is
+            return api_endpoint
         name, mtls, sandbox, googledomain = m.groups()
         if mtls or not googledomain:
             return api_endpoint

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

@@ -161,6 +161,10 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         )
 
         m = mtls_endpoint_re.match(api_endpoint)
+        if m is None:
+            # could not parse api_endpoint; return as-is
+            return api_endpoint
+
         name, mtls, sandbox, googledomain = m.groups()
         if mtls or not googledomain:
             return api_endpoint

gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -146,13 +146,15 @@ def test__get_default_mtls_endpoint():
     sandbox_endpoint = "example.sandbox.googleapis.com"
     sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
     non_googleapi = "api.example.com"
+    custom_endpoint = ".custom"
 
     assert {{ service.client_name }}._get_default_mtls_endpoint(None) is None
     assert {{ service.client_name }}._get_default_mtls_endpoint(api_endpoint) == api_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(api_mtls_endpoint) == api_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_endpoint) == sandbox_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_mtls_endpoint) == sandbox_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(non_googleapi) == non_googleapi
+    assert {{ service.client_name }}._get_default_mtls_endpoint(custom_endpoint) == custom_endpoint
 
 def test__read_environment_variables():
     assert {{ service.client_name }}._read_environment_variables() == (False, "auto", None)