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

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.

Testing Done:
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: https://travis-ci.org/pantsbuild/pants/builds/117770399

Bugs closed: 3073

Reviewed at https://rbcommons.com/s/twitter/r/3596/

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