$fixnull behavior is broken on PostgreSQL's ::addField()

Problem/Motivation

PostgreSQL's Schema::addField() does not properly detect when a field is part of a newly added primary key, so that it does not calculate T $fixnull variable correctly.

This is because a $keys_new variable is inspected but the variable is actually called $new_keys.

Proposed resolution

Because the variable is actually specified as $keys_new by the interface, rewrite usages of $new_keys to $keys_new instead of the other way around.

This will probably require #2974722: Fix dropping of columns in a composite primary key on MariaDB >=10.2.8 and make it consistent across database drivers → for test coverage.