Fix collate and null processing in package constants

[Noremos]

Fix Regression decribed in https://groups.google.com/g/firebird-devel/c/5ehuR18Wemk and https://groups.google.com/g/firebird-devel/c/kze2LTWaeco

[asfernandes]

Why is constant references always marked as nullable?

[Noremos]

To return NULL from the ReferenceNode, the nullable flag is requered. It would be nice to explicitly check the not null state of the field, but as I can see, it does not stored in RDB$FIELD. So I decied to put the nullable flag withou extra checks.

The not null clasue can only be used when the type is a domain:

create domain my_domain int not null;

create or alter package my_package as
begin
    constant my_const my_domain = null;
end;

But in this case an error will appear instanly:

Statement failed, SQLSTATE = 2F000
validation error for CAST, value "*** null ***"
-Error while parsing BLR value of the constant "PUBLIC"."MY_PACKAGE"."MY_CONST"

So, putting nullable flag in any case is safe

[asfernandes]

Every other kind of expressions, include composed ones, do return nullable flag correctly. I see no reason for constants to flag nullable incorrectly.

[dyemanov]

It would be nice to explicitly check the not null state of the field, but as I can see, it does not stored in RDB$FIELD

RDB$FIELDS.RDB$NULL_FLAG ?