Histogram rendering doesn't handle missing data gracefully

[ressy]

Samples with reads present in the file but none matching for that particular sample cause spurious warnings when saving histograms, like:

Warning in min(x) : no non-missing arguments to min; returning Inf
Warning in max(x) : no non-missing arguments to max; returning -Inf

This is because str_hist_render calls things like max/min/range on potentially-empty data frames. The tests curently miss this because they skip rendering the plots.