Allow suppressing of "Enter some keywords." error message when submitting an empty form, path keys=

Problem/Motivation

On mobile views that collapse the search form into a search button, the way Drupal core search worked is you would simply end up on the search page— no error. With Search Exclude, it (reasonably, but unfortunately in this case) shouts at the person that they need to enter keywords before they'd had a chance to.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes