MDL-75170 dml: add sql_order_by_null method

Standardises return patterns of null values across database types
to keep with sorting nullsto the top when ascending, and
to the bottom when descending

lib/dml/pgsql_native_moodle_database.php

@@ -1525,6 +1525,19 @@ class pgsql_native_moodle_database extends moodle_database {
         return "STRING_AGG(" . $this->sql_cast_to_char($field) . ", '{$separator}' {$fieldsort})";
     }
 
+    /**
+     * Returns the SQL text to be used to order by columns, standardising the return
+     * pattern of null values across database types to sort nulls first when ascending
+     * and last when descending.
+     *
+     * @param string $fieldname The name of the field we need to sort by.
+     * @param int $sort An order to sort the results in.
+     * @return string The piece of SQL code to be used in your statement.
+     */
+    public function sql_order_by_null(string $fieldname, int $sort = SORT_ASC): string {
+        return parent::sql_order_by_null($fieldname, $sort) . ' NULLS ' . ($sort == SORT_ASC ? 'FIRST' : 'LAST');
+    }
+
     public function sql_regex_supported() {
         return true;
     }