Fix errors due to iterating over None-types in ivy resolve.

Review Request #3596 - Created March 22, 2016 and submitted

Garrett Malmquist
nhoward_tw, stuhood, zundel
IvyFetchResolveResult._jar_dependencies_for_target was returning
None instead of an empty iterable, which was breaking the for-loop
in IvyInfo.get_resolved_jars_for_coordinates.

This makes _jar_dependencies_for_target return an empty tuple
instead of None.

Manually confirmed that this fixes the problem in our internal repo.
CI confirmed that this didn't break any existing test-cases.

I haven't found a way to repro this in open-source yet (the artifact that was causing ivy to choke internally is an internal-only artifact that happens to be a tarball). I tried adding a test using an arbitrary tarball I found on maven central, but it didn't reproduce the original error.

CI passed here:

Eric Ayers
Eric Ayers
Matt Olsen
Garrett Malmquist
Review request changed

Status: Closed (submitted)

Change Summary:

In 6d6c24446f94a79d35818f14400e80c525362782, Thanks Eric and Matt. Hopefully we can eventually discover a way to regression test this.