If a SQL-standard function body contains an INSERT ... SELECT statement,
any function parameters referenced within the SELECT were always printed
in $N style, rather than using the parameter name if any.  While not
strictly incorrect, this wasn't the intention, and it's inconsistent
with the way that such parameters would be printed in any other kind
of statement.

The cause is that the recursion to get_query_def from
get_insert_query_def neglected to pass down the context->namespaces
list, passing constant NIL instead.  This is a very ancient oversight,
but AFAICT it had no visible consequences before commit e717a9a1
added an outermost namespace with function parameters.  We don't allow
INSERT ... SELECT as a sub-query, except in a top-level WITH clause,
where it couldn't contain any outer references that might need to access
upper namespaces.  So although that's arguably a bug, I don't see any
point in changing it before v14.

In passing, harden the code added to get_parameter by e717a9a1 so that
it won't crash if a PARAM_EXTERN Param appears in an unexpected place.

Per report from Erki Eessaar.  Code fix by me, regression test case
by Masahiko Sawada.

Discussion: https://postgr.es/m/AM9PR01MB8268347BED344848555167FAFE949@AM9PR01MB8268.eurprd01.prod.exchangelabs.com