Fix bare return statements in prepare() to return null

Two early-exit paths used bare return; instead of return null;
to match the documented @return string|null type.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: westonruter

src/wp-includes/class-wpdb.php

@@ -1666,7 +1666,7 @@ public function prepare( $query, ...$args ) {
 				'6.2.0'
 			);
 
-			return;
+			return null;
 		}
 
 		$args_count = count( $args );
@@ -1684,7 +1684,7 @@ public function prepare( $query, ...$args ) {
 					'4.9.0'
 				);
 
-				return;
+				return null;
 			} else {
 				/*
 				 * If we don't have the right number of placeholders,