func_logic: Don't emit warning if both IF branches are empty.

func_logic: Don't emit warning if both IF branches are empty.

The IF function currently emits warnings if both IF branches
are empty. However, there is no actual necessity that either
branch be non-empty as, unlike other conditional applications/
functions, nothing is inherently done with IF, and both
sides could legitimately be empty. The warning is thus turned
into a debug message.

ASTERISK-30243 #close

Change-Id: I5250625dd720f95e1859b5dfb933905d7e7a730e