Make input and additionalInput optional in StepRequestBody

Updated the StepRequestBody class to allow both 'input' and 'additionalInput' to be optional. Added logic in toJson() method to return an empty JSON object if both fields are null.

frontend/lib/models/step_request_body.dart

@@ -1,10 +1,13 @@
 class StepRequestBody {
-  final String input;
+  final String? input;
   final Map<String, dynamic>? additionalInput;
 
   StepRequestBody({required this.input, this.additionalInput});
 
   Map<String, dynamic> toJson() {
+    if (input == null && additionalInput == null) {
+      return {};
+    }
     return {'input': input, 'additional_input': additionalInput};
   }
 }