C API: Make nix_err_msg treat NIX_OK as having no message

The documentation "solved" this by specifying a precondition, but
let's just make it more robust, and not leak irrelevant messages
that might linger.
We don't clear the message when clearing the status, in order to
keep clearing fast; see last_err field doc.

src/libutil-c/nix_api_util_internal.h

@@ -10,6 +10,7 @@
 struct nix_c_context
 {
     nix_err last_err_code = NIX_OK;
+    /** The last error message. Always check last_err_code. This may not have been cleared, so that clearing is fast. */
     std::optional<std::string> last_err = {};
     std::optional<nix::ErrorInfo> info = {};
     std::string name = "";