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/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -1013,12 +1013,11 @@ class Fir2IrVisitor(
         }
         return conversionScope.withWhenSubject(subjectVariable) {
             whenExpression.convertWithOffsets { startOffset, endOffset ->
-                // If the constant true branch has empty body, it won't be converted. Thus, the entire `when` expression is effectively _not_
-                // exhaustive anymore. In that case, coerce the return type of `when` expression to Unit as per the backend expectation.
-                val irBranches = whenExpression.branches.mapNotNullTo(mutableListOf()) { branch ->
-                    branch.takeIf {
-                        it.condition !is FirElseIfTrueCondition || it.result.statements.isNotEmpty()
-                    }?.toIrWhenBranch(whenExpression.typeRef)
+                if (whenExpression.branches.isEmpty()) {
+                    return@convertWithOffsets IrBlockImpl(startOffset, endOffset, irBuiltIns.unitType, origin)
+                }
+                val irBranches = whenExpression.branches.mapTo(mutableListOf()) { branch ->
+                    branch.toIrWhenBranch(whenExpression.typeRef)
                 }
                 if (whenExpression.isProperlyExhaustive && whenExpression.branches.none { it.condition is FirElseIfTrueCondition }) {
                     val irResult = IrCallImpl(

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/ir/Fir2IrTextTestGenerated.java

@@ -2600,6 +2600,12 @@ public class Fir2IrTextTestGenerated extends AbstractFir2IrTextTest {
             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 {

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/ir/LightTreeFir2IrTextTestGenerated.java

@@ -2600,6 +2600,12 @@ public class LightTreeFir2IrTextTestGenerated extends AbstractLightTreeFir2IrTex
             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 {