fix: allow empty location for cluster update (#1258)

Fixes #15311

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: daniel-sanche

packages/google-cloud-bigtable/google/cloud/bigtable/cluster.py

@@ -511,9 +511,11 @@ def delete(self):
     def _to_pb(self):
         """Create cluster proto buff message for API calls"""
         client = self._instance._client
-        location = client.instance_admin_client.common_location_path(
-            client.project, self.location_id
-        )
+        location = None
+        if self.location_id:
+            location = client.instance_admin_client.common_location_path(
+                client.project, self.location_id
+            )
 
         cluster_pb = instance.Cluster(
             location=location,