User input variables in launch config are broken

- Add one of the special `input` to some launch config, like

```
"cwd": "${input:cwd}"
```

- Start the launch config
- Get "Cannot read property 'filter' of undefined"

From https://github.com/microsoft/vscode/blob/6a7d02f2f44446a7eda01abeb1c25122e148d264/src/vs/workbench/services/configurationResolver/browser/configurationResolverService.ts#L231