Support include empty config file. v5.0.173 v6.0.68 (#3768)

SRS supports including another configuration in the include package.
When generating configurations, we can only generate the changed
configurations, while the unchanged configurations are in the fixed
files, for example:

```nginx
listen 1935;
include server.conf;
```

In `server.conf`, we can manage the changing configurations with the
program:

```nginx
http_api { enabled on; }
```

However, during system initialization, we often create an empty
`server.conf`, and the content is generated only after the program
starts, so `server.conf` might be an empty file. This also makes it
convenient to use a script to confirm the existence of this file:

```bash
touch server.conf
```

Currently, SRS does not support empty configurations and will report an
error. This PR is to solve this problem, making it more convenient to
use include.

`TRANS_BY_GPT4`

---------

Co-authored-by: Haibo Chen <495810242@qq.com>

trunk/src/utest/srs_utest_config2.hpp

@@ -0,0 +1,16 @@
+//
+// Copyright (c) 2013-2023 The SRS Authors
+//
+// SPDX-License-Identifier: MIT or MulanPSL-2.0
+//
+
+#ifndef SRS_UTEST_CONFIG2_HPP
+#define SRS_UTEST_CONFIG2_HPP
+
+/*
+#include <srs_utest_config2.hpp>
+*/
+#include <srs_utest_config.hpp>
+
+#endif
+