Allow null values for non-resource image paths.

```
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Picasso (Parent) .................................. SUCCESS [0.713s]
[INFO] Picasso ........................................... SUCCESS [10.499s]
[INFO] Picasso Sample .................................... SUCCESS [7.852s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 19.941s
[INFO] Finished at: Thu Jun 13 10:32:48 PDT 2013
[INFO] Final Memory: 31M/81M
[INFO] ------------------------------------------------------------------------
```