A BUG caused by pandas.Index.map

[zhaozp15]

In MOTAccumulator.merge_event_dataframes （Line 443), we use copy.index.map to update index.
When copy is an empty DataFrame, the size of copy.index is 0.
In this situation, copy.index.map will return an object of type Index rather than MultiIndex.

We can fix it by adding if copy.index.size != 0 before copy.index.map