Fix parsetree representation of XMLTABLE(XMLNAMESPACES(DEFAULT ...)).
The original coding for XMLTABLE thought it could represent a default
namespace by a T_String Value node with a null string pointer. That's
not okay, though; in particular outfuncs.c/readfuncs.c are not on board
with such a representation, meaning you'll get a null pointer crash
if you try to store a view or rule containing this construct.
To fix, change the parsetree representation so that we have a NULL
list element, instead of a bogus Value node.
This isn't really a functional limitation since default XML namespaces
aren't yet implemented in the executor; you'd just get "DEFAULT
namespace is not supported" anyway. But crashes are not nice, so
back-patch to v10 where this syntax was added. Ordinarily we'd consider
a parsetree representation change to be un-backpatchable; but since
existing releases would crash on the way to storing such constructs,
there can't be any existing views/rules to be incompatible with.
Per report from Andrey Lepikhov.
Discussion: https://postgr.es/m/
3690074f-abd2-56a9-144a-
aa5545d7a291@postgrespro.ru