undefined method `supervisees`for nil

https://github.com/rails/solid_queue/blame/c521c6d47d10abeaf076a81d9af29e80eb2b11c7/lib/solid_queue/supervisor.rb#L174

probably should look something like `process&.supervisees.find_by...` or 

```
if registered_process = process.supervisees.find_by(name: terminated_fork.name) rescue false
```