Fix straight to merge when splitting is changed

If splitting is changed through script/pages and there is a
transient output module, the property of disabling
straight to merge is broken by the change which can
cause spurious output in merged area from straight-to-merge
mechanisms.
Merge pull request #4457 from dballesteros7/FixTransientOutputModules

Fix straight to merge when splitting is changed
0.9.49

Signed-off-by: Stephen Foulkes <sfoulkes@fnal.gov>