Fix PL/Python metadata when there is no result

Dear hackers,

Thanks for the work on PLPython result metadata, it is very useful!  I
just came across a crash when trying to access this metadata on the
result of an UPDATE, which obviously cannot return any tuple (unless
you specify a RETURNING clause maybe?).

Please find attached a patch that solves this issue.  Instead of a PG
crash, we get the following message:

ERROR:  plpy.Error: no result fetched

All the best,
--
Jean-Baptiste Quenot