Don't throw error if content is empty

If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

lib/structs/events/messages/unknown.cpp

@@ -22,7 +22,8 @@ from_json(const json &obj, Unknown &content)
 void
 to_json(json &obj, const Unknown &content)
 {
-    obj            = json::parse(content.content);
+    if (!content.content.empty())
+        obj = json::parse(content.content);
     obj["msgtype"] = content.msgtype;
     obj["body"]    = content.body;
     common::apply_relations(obj, content.relations);