Simplify some code in getObjectTypeDescription()

Simplify some code in getObjectTypeDescription()

This routine is designed to never return an empty description or NULL,
providing description fallbacks even if missing objects are accepted,
but it included a code path where this was considered possible.  All the
callers of this routine already consider NULL as not possible, so
change a bit the code to map with the assumptions of the callers, and
add more comments close to the callers of this routine to outline the
behavior expected.

This code is new as of 2a10fdc, so no backpatch is needed.

Discussion: https://postgr.es/m/YMNY6RGPBRCeLmFb@paquier.xyz