JSON query startwith operator has unexpected behavior in the face of NULL value

[Galen Charlton]

As indirectly noted in bug 1745462, the JSON query startwith operator can behave unexpectedly in the face of a NULL/undef argument. For example,

{"where":{"control_set":"1","simple_heading":{"startwith":null},"deleted":"f"},"from":"are","select":{"are":["id"]}}

becomes

SELECT "are".id AS "id" FROM authority.record_entry AS "are" WHERE "are".control_set = '1' AND "are".deleted = 'f' AND "are".simple_heading IS NOT NULL;

A JSON query containing "startwith":null should either be considered a malformed query or, perhaps, turn into "IS NULL".

Evergreen master