PHP Warning: Attempt to read property "display_name" on bool

Reported here - https://github.com/a8cteam51/safety-net/issues/78

```
Warning: Attempt to read property "display_name" on bool in /plugins/sensei-lms/includes/class-sensei-modules.php on line 2653
```

Unfortunately, the original report doesn't specify how to reproduce this but we can likely just harden this code.