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.ir.txt

@@ -0,0 +1,21 @@
+FILE fqName:<root> fileName:/emptyWhen.kt
+  FUN name:foo visibility:public modality:FINAL <> () returnType:kotlin.Unit
+    BLOCK_BODY
+      BLOCK type=kotlin.Unit origin=WHEN
+      VAR name:x type:kotlin.Int [val]
+        CONST Int type=kotlin.Int value=0
+      BLOCK type=kotlin.Unit origin=WHEN
+        VAR IR_TEMPORARY_VARIABLE name:tmp_0 type:kotlin.Int [val]
+          GET_VAR 'val x: kotlin.Int [val] declared in <root>.foo' type=kotlin.Int origin=null
+        WHEN type=kotlin.Unit origin=WHEN
+          BRANCH
+            if: CONST Boolean type=kotlin.Boolean value=true
+            then: BLOCK type=kotlin.Unit origin=null
+      VAR name:z type:kotlin.Unit [val]
+        BLOCK type=kotlin.Unit origin=WHEN
+          VAR IR_TEMPORARY_VARIABLE name:tmp_1 type:kotlin.Int [val]
+            GET_VAR 'val x: kotlin.Int [val] declared in <root>.foo' type=kotlin.Int origin=null
+          WHEN type=kotlin.Unit origin=WHEN
+            BRANCH
+              if: CONST Boolean type=kotlin.Boolean value=true
+              then: BLOCK type=kotlin.Unit origin=null

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 -> {}
+    }
+}

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
+      }
+    }
+  }
+}
+