Previously, if some or all partitions had no partially aggregated path,
we would still try to generate a partially aggregated path for the
parent, leading to assertion failures or wrong answers.

Report by Rajkumar Raghuwanshi.  Patch by Jeevan Chalke, reviewed
by Ashutosh Bapat.  A few changes by me.

Discussion: http://postgr.es/m/CAKcux6=q4+Mw8gOOX16ef6ZMFp9Cve7KWFstUsrDa4GiFaXGUQ@mail.gmail.com