HPCC-12285 Fix set index ranges

Signed-off-by: JamieNoss <james.noss@lexisnexis.com>
I've thought about it, and I think this is wrong, rather than unhelpful.

If you think of the expression
set1 set2

it is effectively taking a list of indexes from the second set, and extracting those elements from the first set.  So myset[3..4]  extracts elements 3.4.   (We actually have something very similar to this that is supported for sets of datasets).

Therefore set[] should evaluate to an EMPTY set of the correct type.

Having said that, until we implement the "set set" syntax properly adding this special case just makes things more confusing, so please restore the previous error.