Remove None value before returning connection names (microsoft#336)

Remove None value when get connection names.

Co-authored-by: Lina Tang <[email protected]>

src/promptflow/promptflow/contracts/flow.py

@@ -515,7 +515,7 @@ def get_connection_names(self):
             tool = self.get_tool(node.tool) or self._tool_loader.load_tool_for_node(node)
             if tool:
                 connection_names.update(self._get_connection_name_from_tool(tool, node).values())
-        return connection_names
+        return set({item for item in connection_names if item})
 
     def get_connection_input_names_for_node(self, node_name):
         """Return connection input names."""