Panic in join against diff table

[timsehn]

Repro steps:

CREATE TABLE mytable (
    id VARCHAR(255) PRIMARY KEY,
    str VARCHAR(255)
);

INSERT INTO mytable (id, str) VALUES
('one', 'aa'),
('two', 'bb');

SELECT * FROM dolt_diff_mytable LEFT JOIN mytable ON id = to_id;

Error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x68 pc=0x105ea5be4]

[timsehn]

Fix is here: #8699