Fix m.relates_to being sent as 'null' when not set in encrypted messages.

Thank you Sorunome for reporting.

src/Olm.cpp

@@ -168,7 +168,8 @@ encrypt_group_message(const std::string &room_id, const std::string &device_id,
 
         // relations shouldn't be encrypted...
         mtx::common::ReplyRelatesTo relation;
-        if (body["content"]["m.relates_to"].contains("m.in_reply_to")) {
+        if (body["content"].contains("m.relates_to") &&
+            body["content"]["m.relates_to"].contains("m.in_reply_to")) {
                 relation = body["content"]["m.relates_to"];
                 body["content"].erase("m.relates_to");
         }