ExceptionMapperBase.toResponse has no protection if toProblem() throws or returns null

[phillip-kruger]

If a subclass's toProblem() implementation throws an exception or returns null, ExceptionMapperBase.toResponse() crashes with an unhelpful error. The original mapped HttpProblem is lost and the user gets an uncontrolled error instead of a proper application/problem+json response.

The method should catch exceptions from toProblem() and handle null returns by falling back to a generic 500 problem response.