Tidy-up unneeded NULL parameter checks from SQL function

Tidy-up unneeded NULL parameter checks from SQL function

This function is marked as strict, so we can safely remove the checks
checking for NULL input parameters.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAApHDvqiN0+mbooUOSCDALc=GoM8DmTbCdvwnCwak6Wb2O1ZJA@mail.gmail.com