Apply the solution adopted in commit dcb7d3ca (ie, explicitly
don't call memcmp for a zero-length comparison) to func_get_detail()
as well, removing one other place where we were passing an
uninitialized array to a parse_func.c entry point.

Discussion: https://postgr.es/m/MN2PR18MB2927F24692485D754794F01BE3740@MN2PR18MB2927.namprd18.prod.outlook.com
Discussion: https://postgr.es/m/MN2PR18MB2927F6873DF2774A505AC298E3740@MN2PR18MB2927.namprd18.prod.outlook.com