D-10651 - Search '%2B' returns a 500, should return a 400

- Empty search categories (any plus with no term following it, or is immediately followed by a plus) now return a 400

NOTE:  previously, any search query with a trailing '+' threw a 500, however, a search of '++term1' would return, with only 'term1' categories, not a 500.

This PR now has the following return a 400:
  * '+'
  * '+cat1+'
  * '++cat1'

Is this a no-no because some customer might already being using #3 and getting results, and this would be a 'regression'?