refactor: improve default global variable retrieval logic for better clarity

Author: wxg0103

apps/application/flow/step_node/start_node/impl/base_start_node.py

@@ -17,9 +17,11 @@
 
 
 def get_default_global_variable(input_field_list: List):
-    return {item.get('variable'): item.get('default_value') for item in input_field_list if
-            item.get('default_value', None) is not None}
-
+    return {
+        item.get('variable') or item.get('field'): item.get('default_value')
+        for item in input_field_list
+        if item.get('default_value', None) is not None
+    }
 
 def get_global_variable(node):
     body = node.workflow_manage.get_body()