RichDLLs: Only fill plot for DLL(X-Y) if one or another is above threshold

Following a discussion with @fbetti on small variations seen in these plots as part of the DQ checks, I have decided to filter out entries where for DLL(X-Y) both X and Y are below threshold. These result in a spike at zero in the distribution. The relative size of this spike is not really a measure of the RICH performance, but more a reflection of the track momentum distribution, and it appears it is quite sensitive to data taking conditions.

This will require ref updates