Docs: Replace void with null in union return types in class-wpdb.php

Fixes #64703.
See #64694.

Co-Authored-By: xateman

Author: apermo

src/wp-includes/class-wpdb.php

@@ -1453,11 +1453,11 @@ private function _escape_identifier_value( $identifier ) {
 	 *                             individual arguments.
 	 * @param mixed       ...$args Further variables to substitute into the query's placeholders
 	 *                             if being called with individual arguments.
-	 * @return string|void Sanitized query string, if there is a query to prepare.
+	 * @return string|null Sanitized query string, if there is a query to prepare.
 	 */
 	public function prepare( $query, ...$args ) {
 		if ( is_null( $query ) ) {
-			return;
+			return null;
 		}
 
 		/*
@@ -2117,7 +2117,7 @@ public function parse_db_host( $host ) {
 	 * @since 3.9.0
 	 *
 	 * @param bool $allow_bail Optional. Allows the function to bail. Default true.
-	 * @return bool|void True if the connection is up.
+	 * @return bool True if the connection is up.
 	 */
 	public function check_connection( $allow_bail = true ) {
 		// Check if the connection is alive.
@@ -3056,7 +3056,7 @@ public function get_var( $query = null, $x = 0, $y = 0 ) {
 	 *                            correspond to an stdClass object, an associative array, or a numeric array,
 	 *                            respectively. Default OBJECT.
 	 * @param int         $y      Optional. Row to return. Indexed from 0. Default 0.
-	 * @return array|object|null|void Database query result in format specified by $output or null on failure.
+	 * @return array|object|null Database query result in format specified by $output or null on failure.
 	 */
 	public function get_row( $query = null, $output = OBJECT, $y = 0 ) {
 		$this->func_call = "\$db->get_row(\"$query\",$output,$y)";