No count image is returned when interpolating if the target date range has no images (but the interpolation date range does)

[cgmorton]

This is happening for a number of CONUS monthly MGRS tiles including 10TDQ_20170401_20170430 and 10TDR_20170401_20170430. This seems to be happening because there are images in March and May so ETf is being interpolated for April, but since there are no images the "aggregation_collection", no count image is built.

One possible fix is adding an extra "zero" image to the aggregation collection so that it cannot be empty. I've implemented this in the "no-count" branch and added a test and it seems to be working. For this initial fix, I am adding a constant image of zero and then assuming the image will be clipped as needed but it might be better to use the etf or etr images as a mask instead.