Harfbuzz assumes the length of the item is at least 1.

If the length is zero, then it will clobber memory at index -1
into the log_clusters array.

Explicitly handle the cases where the entire string or a single
run might have a length of 0.

Bug: 5705479
Change-Id: Ibbd3a4edcb7e1cad09c34091b42bb315776ea558