[BEAM-1392] DoFn teardown not called on empty partitions

[aviemzur]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

[aviemzur]

R: @amitsela

[amitsela]

LGTM.

[coveralls]

Coverage increased (+0.01%) to 69.691% when pulling cf1ea05 on aviemzur:invoke-teardown-on-empty-partition into 6e220bb on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/7087/
--none--

[dhalperi]

Driveby: nice catch!

[amitsela]

Yeah.. we had a nasty "too many open files" when running against sparse topics..