added tests for branching in tables _join

[rwmehta]

[X] Wrote test for feature
[ ] Added changes to CHANGELOG.md

Changes proposed:
Added tests for if and if not branches in tables _join method as requested in issue #476 .
Ensures the method returns None when either table has no rows, and matches the label when "other_label" is not provided.

[coveralls]

coverage: 94.976%. remained the same
when pulling 572b08f on rwmehta:rwmehta/test-coverage-table-join
into 0c82a19 on data-8:master.

[davidwagner]

This doesn't seem to increase test coverage. I think these cases are already covered by other pre-existing test cases: empty tables by test_join_no_rows(), and no 3rd argument to join by many other test cases. But OK, why not, more tests can't hurt. Thanks for the contribution.