FIR2IR: never generate empty when #KT-55459 Fixed

In details, this commit changes the following:
- it converts FIR when without branches to empty IR block without when
- it doesn't drop empty else branches in when anymore

compiler/testData/codegen/box/when/emptyWhen.kt

@@ -1,6 +1,3 @@
-// KT-55459
-// IGNORE_BACKEND_K2: NATIVE
-
 enum class A { X1, X2 }
 
 fun box(): String {