Implement relational NULL/Nothing for join for in-memory tables

[GregoryTravis]

Pull Request Description

Implements relational NULL for join, for all Join_Kinds.

Checklist

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• The documentation has been updated, if necessary.
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[AdRiley]

Not something to fix for this PR, but in general I find

        t1 = table_builder [["X", ["A", Nothing, "a", Nothing, "ą", "b"]], ["Y", [0, 1, 2, 3, 4, 5]]]
        t2 = table_builder [["X", ["a", Nothing, Nothing, "b"]], ["Z", [10, 20, 30, 50]]]

really hard to read. To the point that I want to write it out on paper.

Do we have a better way of defining tables from text?

[GregoryTravis]

Not something to fix for this PR, but in general I find

        t1 = table_builder [["X", ["A", Nothing, "a", Nothing, "ą", "b"]], ["Y", [0, 1, 2, 3, 4, 5]]]
        t2 = table_builder [["X", ["a", Nothing, Nothing, "b"]], ["Z", [10, 20, 30, 50]]]

really hard to read. To the point that I want to write it out on paper.

Do we have a better way of defining tables from text?

It's very tempting to write something to parse an ascii table.

But we could encourage the practice of defining tables as rows, not columns, that's somewhat easier to read.