There is an optimization for searched CASE where values are of boolean
type. It was converting the expression like

    CASE
        WHEN X THEN A
        WHEN Y THEN B
        ..
        [ ELSE D ]
    END

into

    (X AND A)
        OR (Y AND NOT X AND B)
        [ OR (NOT (X OR Y) AND D) ]

This had the following problems

- does not work for nullable conditions. If X is nullable, we cannot use
  NOT (X) to compliment it. We need to use `X IS DISTINCT FROM true`
- it does not work correctly when some conditions are nullable and other
  values are false. E.g. X=NULL, A=true, Y=NULL, B=true, D=false, the
  CASE should return false, but the boolean expression will simplify to
  `(NULL AND ..) OR (NULL AND ..) OR (false)` which is NULL, not false
  - thus we use `X` for truthness check of `X`, we need to test `X IS
    NOT DISTINCT FROM true`
- it did not work correctly when default D is missing, but conditions
  do not evaluate to NULL. CASE's result should be NULL but was false.

This commit fixes that optimization.