HPCC-10537 Allow LOCAL on MERGEJOIN, JOIN([]) and MERGE([])

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6652 from ghalliday/issue10537a

HPCC-10537 Allow LOCAL on MERGEJOIN, JOIN([]) and MERGE([])

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>