Text formatter: don't print empty space when no msg

The field system is great, it allows clearer logs, however the
result is that there may be nothing in the `entry.Data["msg"]`

Example:

```go
logger.WithFields(logrus.Fields{
	"event": "doing-smth", "error": err, "object": obj,
}).Error()
```

But in this case 45 characters are empty when using the text formatter.

![screenshot from 2014-03-29 21 59 43](https://cloud.githubusercontent.com/assets/329969/2559803/934ce51a-b78d-11e3-98c2-eb5436540b31.png)

After this patch, when entry.Data["msg"] is empty, we get that:

![screenshot from 2014-03-29 22 01 27](https://cloud.githubusercontent.com/assets/329969/2559806/b8f35cb8-b78d-11e3-9645-34aef89c0efc.png) 

What do you think of that, how could it be improved?

Thank you