ROOTObjectWriter: Only write to tree if new, otherwise fill new branch

The fix submitted in !224 (merged) for the missing reference issue only addressed the case, when a completely new type of message was received. However, there can also be the case where the message type has been seen before (the tree exists) but this specific detector (the branch within the tree) has not produced this message yet.

In this case we were wrongly filling the whole tree with empty leafs, essentially creating an offset of last_event_ entries.

This MR addresses this by checking if the tree actually existed already and only a new branch is added. In this case, only the new branch is filled.

This fixes #163 (closed)