It can raise the same kind of error as other validation for consistency.
If it's a common pattern in your project, write a helper.
Like no other form validation library, simply write in English your requirements inside your form HTML tags, Parsley will do the rest!
No need to write even a single Java Script line for simple form validation.
One required param mutually exclusive with an optional param will mean the latter is never valid.
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To add to the complication: The main key cant be empty (null or '') I've often come across scenarios like this and wrote a bunch of conditional statements to test this.
I don't have access to the JS so I cant modify it and therefore I'm stuck with this situation.