Only use array_merge() on subproject_ids when required

Previously, the subproject merging code unconditionally called array_merge()
for each project, even if there were no subproject_ids to merge.

For a big number of projects, these array_merge() invocations are expensive.
Tweak this to only call array_merge() if there are subproject_ids to merge.

Signed-off-by: David Hicks d@hx.id.au