Support empty golden config file for multi-asic (#3670)

What I did
config_sonic_basedon_testbed.yml creates an empty golden_config_db.json file under /etc/sonic for non mx platforms.
When this empty file exist db_migration fails for all asics for non mx platforms.

How I did it
Update db_migrator to support empty golden config file.

How to verify it
Run unit test and end to end test

scripts/db_migrator.py

@@ -127,7 +127,7 @@ def generate_config_src(self, ns):
                             config_namespace = "localhost"
                         else:
                             config_namespace = ns
-                        golden_config_data = golden_data[config_namespace]
+                        golden_config_data = golden_data.get(config_namespace, None)
         except Exception as e:
             log.log_error('Caught exception while trying to load golden config: ' + str(e))
             pass

tests/db_migrator_test.py

@@ -853,6 +853,13 @@ def test_golden_config_hostname(self):
         # hostname is from golden_config_db.json
         assert hostname == 'SONiC-Golden-Config'
 
+    def test_golden_config_ns(self):
+        # golden_config_db.json.test has no namespace
+        import db_migrator
+        dbmgtr = db_migrator.DBMigrator("asic0")
+        result = json.dumps(dbmgtr.config_src_data)
+        assert 'SONiC-Golden-Config' not in result
+
 class TestGoldenConfigInvalid(object):
     @classmethod
     def setup_class(cls):