Hide unzip warning when zip file is empty

unzip prints a warning and exits with code 1 when a zip file is empty.
Pipe the output of unzip through grep -v to hide the warning,
using PIPESTATUS to preserve the exit code of unzip, and then
filter out exit codes that are not 0 or 1.

Test: m
Change-Id: If2cf725fc3315d5996d5a7190288df46d84a59e2