Why can't a Spring Data Specification return a null Predicate? Would't it be sensible to not add it to criteria query if Specification.toPredicate returns null?

I have the following use case:

String q = "<some search query>" // this can be something like "Davy Jones" or "31-12-2011"
Page<Receipt> page = receiptRepository.findAll(where(receiptDateEquals( q)).or(customerNameContains(q)), pageable);

Think about q as a generic query string submitted via a web search form.
Then in the receiptDateEquals specification I'm converting the search query to a date, if it succeeds fine and return the appropriate predicate, otherwise return no predicate / null.

It's off course debatable if I should the date conversion in the specification or in the controller itself.
I would like to opt for a central place in the specification itself so calling controller actions don't have to take care.

That's why I wonder why the Sepcifciation.toPredicate can'y just return null if needed.