sql: allow CAST operation from REAL to BOOLEAN

It was decided that all explicit casts for which we can come up with
meaningful semantics should work. If a user requests an explicit cast,
he/she most probably knows what they are doing.
CAST from REAL to BOOLEAN is disallowed by ANSI rules. However, we allow
CAST from INT to BOOLEAN which is also prohibited by ANSI. So, basically
it is possible to covert REAL to BOOLEAN in two steps:

SELECT CAST(CAST(1.123 AS INT) AS BOOLEAN);

For the reason mentioned above, now we allow straight CAST from REAL to
BOOLEAN. Anything different from 0.0 is evaluated to TRUE.

Part of #4229