fix: use default values for optional workflow input variables (#28546) (#28527)

Co-authored-by: Claude <noreply@anthropic.com>

api/core/app/apps/base_app_generator.py

@@ -93,7 +93,11 @@ class BaseAppGenerator:
         if value is None:
             if variable_entity.required:
                 raise ValueError(f"{variable_entity.variable} is required in input form")
-            return value
+            # Use default value and continue validation to ensure type conversion
+            value = variable_entity.default
+            # If default is also None, return None directly
+            if value is None:
+                return None
 
         if variable_entity.type in {
             VariableEntityType.TEXT_INPUT,