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

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).