Redirect if get parameter is empty

We are building a sign-up form for a gym. We have a page that contains a form for the sign-up. In there we have a hidden field to determine the contract that the user selected and it is pre-populated by the get parameter.

For example:

/deals/ -> The page for the different contracts

From here I click on my selection and I go to the form.

/deals/sign-up/ -> The page with the sign-up form

After I click on contract 1 on the deals page, my structure looks like this:

/deals/sign-up/?contract=1 -> The form knows now that we selected contract one.

What we try to achieve now is that in case someone enters directly to /deals/sign-up/ he gets redirected to /deals/ since he has not made a selection yet and the form does not know which contract he chose.

So what we need is a redirection that detects if there is no get parameter and then send us back to the /deals/ page.

I hope it was understandable 😀

Thank you very much for your help!