CoroutineTransformerMethodVisitor: fix potential type mismatch

The fixed code compiles properly only due to null smart cast.
Without it, e.g. in FIR, we'd have type mismatch error for this return.

compiler/backend/src/org/jetbrains/kotlin/codegen/coroutines/CoroutineTransformerMethodVisitor.kt

@@ -999,7 +999,7 @@ class CoroutineTransformerMethodVisitor(
                 else -> next = next.next
             }
         }
-        return next
+        return null
     }
 
     // It's necessary to preserve some sensible invariants like there should be no jump in the middle of try-catch-block