Add a unit test for join method coverage

[joshu4lim]

[O] Wrote test for feature
[ ] Added changes to CHANGELOG.md
[ ] Bumped version number (delete if unneeded)

Changes proposed:
Hello, I'm a student and a first-time contributor.
One of the coverage missing in 'tables.py' is in the '_join' internal method, in lines 2039-2040 where it checks for an empty table. However, the 'join' method for external interface already has these checks in lines 2024-2025, preventing those lines from being covered. This makes it difficult to test '_join' by just calling 'join' as discussed in this PR.

In this unit test, I decided to modify the 'join' method to skip those checks and allow '_join' to be covered. This way, the functionality of _join can be tested without directly invoking it.

Please let me know if any changes are needed to improve this implementation.

[adnanhemani]

I'm not really in favor of testing in such roundabout ways. If we have dead code, I would much rather remove it instead.

[joshu4lim]

Removed the proposed roundabout test case and removed the dead code inside the _join internal method on lines 2039-2040. Also removed lines 2041-2042 because it would also be dead code based on the same reasoning for removing L2039-2040.

[davidwagner]

@adnanhemani I'll let you comment, but it looks reasonable to me.