What is the shader source string after calling glShaderSource with count=0

What should be in `buf` when the code below is executed? The second call to `glShaderSource` should not generate an error, so it should presumably replace the existing shader source with... nothing?

```
const GLchar *source = "/* Testing 1... 2... 3... */";
const GLchar *junk = "/* Just some junk. */";

glShaderSource(shader, 1, &source, NULL);

glShaderSource(shader, 0, &junk, NULL);

GLchar buf[512];
GLsizei length;

glGetShaderSource(shader, sizeof(buf), &length, buf);
```

See also https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/521 and https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10477.