Timepix: Fix error when no centroid could be computed

When a train does not yield centroid data (i.e. only single pixels lit up) the function empty_centroid_data() is called to produce an empty dataset instead. The returned dictionary missed a key that the centroid data dictionary usually contains and subsequently raises an error. This is fixed here.

The error can be reproduced and the fix tested with run 184 in proposal 900332.