Add test case to tables where join returns none when no values match.

[AlonBaker]

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

Changes proposed:
Adds test_join_no_matching_values to tests/test_tables.py. It covers the case where Table.join returns None because no join values match, even though both tables have rows.

The existing tests that check for None (test_join_no_rows, test_join_with_empty_self and test_join_with_empty_other) all use an empty table. Those return early from the num_rows == 0 check in join, before _join_helper runs, so the if not joined_rows: return None branch in _join_helper had no test. The new test reaches that branch for both a single-column join and a multi-column join.

Addresses Issue #476