There is no option for that in the workflow - that’s what the option ‘this input should not be empty’ on the input is for.
With that option checked, If the input’s value is used in the workflow but the input is empty, the workflow won’t run and the input will be shown with a red outline to indicate it needs to be filled in.
Of course, you could manually create the same process with your own custom states and conditionals for a more customizable UX, but it’s simpler to use the built in feature.
As I said, it only works when the input values are used in the workflow itself, so you just need to make sure your ‘Next’ button workflow references the input values in some way. You could set some custom states, or you can just use a conditional on a workflow step that references the input’s values (i.e. only when input value is not empty).
That will force the workflow to check the inputs aren’t empty, and highlight them if they are.