issue #223: check if args is null in Ln.x(String format, Object... args)

Should be merged ASAP
For `Ln.i("display string: %s", null)` it seems like we would print out:

```
"display string: %s"
```

in the logs.  This seems confusing.  What I've seen other systems do is print out "(null)", eg:

```
display string: (null)
```

This seems like it would be more clear to the reader of the log.