In error message if "actual" is missing indicate it might be `nil`

I think we can make the error message more clear if `actual` appears to be missing. While it may actually be missing, it could also have returned `nil` and now appears to be missing.

Currently:
```
The assertion table must include 'expected' and 'actual'. Missing: 'actual'
```

but yeah, indicating that it might be returning `nil` could help the test writer from some confusion.