flash: add and implemented flash_is_erased() for sandbox flash

source/drivers/sandbox_flash.c

@@ -32,6 +32,7 @@ static struct flash_ops stdio_flash_ops = {
 	.read = stdio_flash_read,
 	.write = stdio_flash_write,
 	.copy = stdio_flash_copy,
+	.is_erased = default_flash_is_erased,
 	.erase = stdio_flash_erase,
 };