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
+ifself.location_id:
+location=client.instance_admin_client.common_location_path(
+client.project, self.location_id
+ )
 
 cluster_pb=instance.Cluster(
 location=location,