Add explicit return null to get_col_info() fallthrough path

When col_info is empty, the method fell through without an explicit
return statement.

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

Author: westonruter

src/wp-includes/class-wpdb.php

@@ -3905,6 +3905,8 @@ public function get_col_info( $info_type = 'name', $col_offset = -1 ) {
 				return $this->col_info[ $col_offset ]->{$info_type};
 			}
 		}
+
+		return null;
 	}
 
 	/**