fix: adding support to initialize with empty metadata columns (#99)

* Adding null check on the metadata columns

* style: formatting

Author: gauravpurohit06

src/langchain_google_spanner/vector_store.py

@@ -383,7 +383,10 @@ def _generate_sql(
                 )
 
         configs = [id_column, content_column, embedding_column]
-        configs.extend(column_configs)
+
+        if column_configs is not None:
+            configs.extend(column_configs)
+
         column_configs = configs
 
         if primary_key is None: