Fix category_get_id_by_name() doesn't find category

Commit ef0bbb87aecdc54e2b9112bf9f3a11a592dfdc4d introduced a regression.
Testing on the return value as if it were a boolean triggers an error
ERROR_CATEGORY_NOT_FOUND_FOR_PROJECT even if the category exists.

Now we do a strict check === false to ensure the error is only triggered
when the query returns no rows.