Blank address invoice

Now that we're allowing billing contact addresses to be blank, if address is blank, don't include any text in the Bill To field.

Before:
![screen shot 2014-04-01 at 12 54 22 pm](https://cloud.githubusercontent.com/assets/1757035/2581957/715c3004-b9be-11e3-8550-80fbe2f8197c.png)

After:
![screen shot 2014-04-01 at 12 54 13 pm](https://cloud.githubusercontent.com/assets/1757035/2581958/752fb34a-b9be-11e3-8441-a532f1344e5d.png)