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/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/ir/IrTextTestGenerated.java

@@ -2600,6 +2600,12 @@ public class IrTextTestGenerated extends AbstractIrTextTest {
             runTest("compiler/testData/ir/irText/firProblems/deprecated.kt");
         }
 
+        @Test
+        @TestMetadata("emptyWhen.kt")
+        public void testEmptyWhen() throws Exception {
+            runTest("compiler/testData/ir/irText/firProblems/emptyWhen.kt");
+        }
+
         @Test
         @TestMetadata("Fir2IrClassifierStorage.kt")
         public void testFir2IrClassifierStorage() throws Exception {