Core: Fix incorrect searched CASE optimization

[findepi]

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.

• Fixes Invalid query result when searched CASE has nullable condition and boolean result #14343

[alamb]

Thanks @findepi -- this looks good to me