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.txt

@@ -0,0 +1,20 @@
+fun foo() {
+  { // BLOCK
+  }
+  val x: Int = 0
+  { // BLOCK
+    val tmp0_subject: Int = x
+    when {
+      else -> { // BLOCK
+      }
+    }
+  }
+  val z: Unit = { // BLOCK
+    val tmp1_subject: Int = x
+    when {
+      else -> { // BLOCK
+      }
+    }
+  }
+}
+