Fir2Ir converter: fix exception on FirUnitExpression

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

@@ -792,6 +792,7 @@ internal class Fir2IrVisitor(
 
     private fun FirStatement.toIrStatement(): IrStatement? {
         if (this is FirTypeAlias) return null
+        if (this is FirUnitExpression) return toIrExpression()
         return accept(this@Fir2IrVisitor, null) as IrStatement
     }