HPCC-10309 Fix bug where EXISTS(a+b+c) evaluated as EXISTS(c)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5070 from ghalliday/issue10309

HPCC-10309 Fix bug where EXISTS(a+b+c) evaluated as EXISTS(c)

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>