Hi,
Do you want to do like following?
1. User inputs any value -> Use the value on next action.
2. User doesn't input (pending the action) -> Set NULL to the parameter and run next action automatically.
I suppose it cannot be implemented because timeout value is not existed for User Input parameter.
I'm not sure your requirement is satisfied, but my suggestion is
1. User has the value -> Create a file and write the value to the file instead of User Input parameter.
2. Use "Check if file or folder exists" action to check if user could input the value.
3. Use "Read String from Text File" action to check the value if the file is existed.
4. If the file is not existed, you can run another action in your flow.
If you need, please raise your idea as an enhancement request.
Regards
Yas