Allow 0-byte files in full OTAs.

Currently, the writeProcessFunction fails when
there are zero bytes to write, potentially returning
errno from a previous operation, or hanging
indefinitely while it waits for a >0 result on a
write of size 0.  This happens when the output
file is intended to be zero bytes in size.

Change-Id: Ib3cfcaf66d82942bc89e5f5c64697862403b38da