fix add_cols_to_delta_table

Author: bubriks

python/hsfs/engine/spark.py

@@ -1273,16 +1273,18 @@ def save_empty_dataframe(self, feature_group, new_features=None):
         )
 
     def add_cols_to_delta_table(self, feature_group, new_features):
-        new_features_map = {}
-        if isinstance(new_features, list):
-            for new_feature in new_features:
-                new_features_map[new_feature.name] = lit("").cast(new_feature.type)
-        else:
-            new_features_map[new_features.name] = lit("").cast(new_features.type)
-
-        self._spark_session.read.format("delta").load(
+        dataframe = self._spark_session.read.format("delta").load(
             feature_group.get_uri()
-        ).withColumns(new_features_map).limit(0).write.format("delta").mode(
+        )
+
+        if (new_features is not None):
+            if isinstance(new_features, list):
+                for new_feature in new_features:
+                    dataframe = dataframe.withColumn(new_feature.name, lit("").cast(new_feature.type))
+            else:
+                dataframe = dataframe.withColumn(new_features.name, lit("").cast(new_features.type))
+
+        dataframe.limit(0).write.format("delta").mode(
             "append"
         ).option("mergeSchema", "true").option(
             "spark.databricks.delta.schema.autoMerge.enabled", "true"