Want to make a field mandatory if that field has any values other wise it should be optional

[Basudev Behera]

We want to make a field mandatory if that field has any values in dropdown so that user will select any values and if that field does not have any value in drop down then that should be optional.

Please help me to achieve the above scenario.

[Joe Pitt]

You'd have to run some kind of script. I can't help, but others will probably want to know the following:

How would the field change from having valued to not having values in the dropdown list?

Or are you talking about different fields?

Are you populating the dropdown list dynamically via a script or plugin? 

Is this on the create page or in a transition?