Fix the treatment of unmatched cTaus in HLTSeeding
Instead of discarding eTaus which have no matching jTaus as a cTau candidate, treat them as a perfectly-isolated cTau candidate. This is to match the firmware implementation of the matching.
Also fix a bug noticed while changing the code where a signed integer eTau.iEta()
(which has a value between -25 and 24) was implicitly casted to unsigned integer.
Closes ATR-25927