Add fastpaths for when no objects are found

Add fastpaths for when no objects are found

If there are no objects found, there is no reason to inspect the
result columns and mallocing a zero-sized  (which will be 1 byte
in reality) heap buffer for it.  Add a fast-path for immediately
returning like how other object inspection functions are already
doing it.

Reviewed-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/C2F05B3C-1414-45DD-AE09-6FEE4D0F89BD@yesql.se