plugins: don't hand empty strings for unset options.

This was deeply surprising to me; there's a difference between a value not being
specified, and it being specified as "".

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/PLUGINS.md

@@ -40,7 +40,7 @@ methods are required:
  - `getmanifest` asks the plugin for command line options and JSON-RPC
    commands that should be passed through
  - `init` is called after the command line options have been
-   parsed and passes them through with the real values. This is also
+   parsed and passes them through with the real values (if specified). This is also
    the signal that `lightningd`'s JSON-RPC over Unix Socket is now up
    and ready to receive incoming requests from the plugin.
 

lightningd/plugin.c

@@ -1000,8 +1000,7 @@ static void plugin_config(struct plugin *plugin)
 	list_for_each(&plugin->plugin_opts, opt, list) {
 		/* Trim the `--` that we added before */
 		name = opt->name + 2;
-		if (!opt->value)
-			opt->value = "";
+		if (opt->value)
 			json_add_string(req->stream, name, opt->value);
 	}
 	json_object_end(req->stream); /* end of .params.options */