ResultRow#getSize() always returns -1

just had a failing test due to the fact that my code was
trying to find out if the query produce any result before
starting to look over the result entries.

that didn't work since ResultRow#getSize() always returned -1.

if fixed my problem by just getting rid of ResultRow#getSize().
i would suggest to either

• implement getSize() for 'nothing-found' and/or a few results found
• drop the method from the OAK interface altogether if we are never
  going to implement
• add #isEmpty for those cases were someone just wanted to know if a
  query found something without the need of knowing the exact number.