Mark ob_start callback parameter nullable

Author: haszi

ext/standard/basic_functions.stub.php

@@ -1533,7 +1533,7 @@ function header_register_callback(callable $callback): bool {}
 
 /* main/output.c */
 
-/** @param callable $callback */
+/** @param callable|null $callback */
 function ob_start($callback = null, int $chunk_size = 0, int $flags = PHP_OUTPUT_HANDLER_STDFLAGS): bool {}
 
 function ob_flush(): bool {}