commands/helpers.rb: Verify network-cache is non-empty
commands/helpers.rb: Verify network-cache is non-empty

In some circumstances, .git/network-cahche exists but is empty.
This causes json to throw errors when it attempts to parse the
file.  Rather than just verifying that .git/network-cache exists,
we also verify that its size is non-zero.

bfg-repo-cleaner-demos/github-gem-after-bfg