fix: incorrect equality cols for Eq

- In case we have equality between columns
of the same table, `eq_cols_from_eq` should
return empty equality cols, but it returned
None, which led to wrong motion. Fixed that.
- It was found after a merge tuples transform
fix, in front_sql_join_single_left_5 test
where now there are two equalities instead of one.
Earlier, there always was one equality because
merge tuples produced a single (..) = (..)
term for an and-chain. And this test didn't work,
because this term contained one equality pair.
Now there are two terms: (..) = (..) and (..)=(..)
and one of them does not contain any equality
pairs.