MDL-75977 xmldb: Avoid using null params on built-in DB functions

This changes a couple of cases, in postgres and oracle where we
were trying to calculate strlen() on null values (that causes a
PHP 8.1 warning).

Also, at the same time, fixing another case in xmldb_field, it
was detected that it had a bug around precision (being set to the
value of type), being used as object property, when it's not, so
it was also fixed by moving the code logic a little bit.

Note that the bug has been there since inception because there
isn't any code in Moodle using that ->precision property. It
was just detected thanks to PHP 8.2, but that's another story. :-)

Verified that with the patch:
- All DBs can be installed (phpunit install)
- Both DDL and DML pass without any ddl/dml warning (in all DBs).

lib/dml/pgsql_native_moodle_database.php

@@ -545,7 +545,7 @@ class pgsql_native_moodle_database extends moodle_database {
         $tablename = $this->prefix.$table;
 
         $sql = "SELECT a.attnum, a.attname AS field, t.typname AS type, a.attlen, a.atttypmod, a.attnotnull, a.atthasdef,
-                       CASE WHEN a.atthasdef THEN pg_catalog.pg_get_expr(d.adbin, d.adrelid) END AS adsrc
+                       CASE WHEN a.atthasdef THEN pg_catalog.pg_get_expr(d.adbin, d.adrelid) ELSE '' END AS adsrc
                   FROM pg_catalog.pg_class c
                   JOIN pg_catalog.pg_namespace as ns ON ns.oid = c.relnamespace
                   JOIN pg_catalog.pg_attribute a ON a.attrelid = c.oid