34755: distsqlrun: sanity check sortTopKProcessor's parameters r=jordanlewis a=asubiotto

- Change k to a uint64
- Error with a zero k value

We've seen a bug in the wild #33225 where a sortTopKProcessor could have
possibly been created with an invalid k value. This commit ensures that this
state does not crash a server by being overly cautious about the
sorter's k parameter.

Release note: None

Co-authored-by: Alfonso Subiotto Marqués <[email protected]>