js codegen bug for  `case` statement with just `else` branch
### Description

```nim
case 0
else: discard
```
generates
```js
else {}
```
whithout preceding `if`

### Nim Version

Nim Compiler Version 2.0.8 [Linux: amd64]
Compiled at 2024-07-03
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: 5935c3bfa9fec6505394867b23510eb5cbab3dbf
active boot switches: -d:release

### Current Output

_No response_

### Expected Output

_No response_

### Known Workarounds

_No response_

### Additional Information

_No response_