HPCC-9072 Fix issue with null act and multiple outputs

Thor would hit assert(outLink), if it introduced a nullAct
in place of a splitter. The assert was hit when one
of the pulling activities requested an output >0 from the nullAct.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge branch 'hpcc-9072' into candidate-3.10.8

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>