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/ir/irText/firProblems/emptyWhen.kt

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+fun foo() {
+    when {}
+    val x = 0
+    when (x) {
+        else -> {}
+    }
+    val z = when (x) {
+        else -> {}
+    }
+}