Output handlers: Concatenate traceback only if it is not nil.

If there are errors with the tests, the traceback might be nil, which causes a string concatenation to fail if the `verbose` option is set:

```
./usr/share/lua/5.1/busted/outputHandlers/utfTerminal.lua:49: attempt to concatenate field 'traceback' (a nil value)
```

Checking if the traceback is nil before concatenation fixes that.