Rt can be >0 even when there are zero cases

[macfarlandian]

This has been discussed before but doesn't seem to be ticketed; it's a more prominent issue once shadow data is in place.

Sending repeat values to the Rt function seem to produce some weird artifacts, such that the number may start going up. Any repeat value seems to trigger this behavior, though the most striking example is when you send the function case history that consists of all zeros:

It's not clear whether this is a bug in the model code or just a function of the fact that our data will not always fit the profile our model expects; we either need to update the model so that repeat case counts properly "trend toward zero" as we would expect, or detect conditions like this that we know will break our model and handle them some other way.

Possibly related to / overlapping with #449