#13 outer joins with constant values don't work

[Alan Knight]

If you do an outer join against something with a constant
value in it, then you don't get results back.
- first, asIndependentJoins is splitting up the clause and
losing the outerJoinness.
- beyond that, the constant test causes the join to be
effectively non-outer anyway.

We really need to detect this and generate a
(const IS NULL OR const = constValue)