Have LookupFuncName accept NULL argtypes for 0 args

Have LookupFuncName accept NULL argtypes for 0 args

Prior to this change, it requires to be passed a valid pointer just to
be able to pass it to a zero-byte memcmp, per 0a52d378b03b.  Given the
strange resulting code in callsites, it seems better to test for the
case specifically and remove the requirement.

Reported-by: Ranier Vilela
Discussion: https://postgr.es/m/MN2PR18MB2927F24692485D754794F01BE3740@MN2PR18MB2927.namprd18.prod.outlook.com
Discussion: https://postgr.es/m/MN2PR18MB2927F6873DF2774A505AC298E3740@MN2PR18MB2927.namprd18.prod.outlook.com