Refactored image_fu helper to only use geometry when not nil.

This was causing issues with the rickrockstar example on at least Refinery v2.1.5.  I'm fixing this in master and then back-porting it and releasing Refinery v2.1.6.

What was happening is that when using the following:

```erb
<%= image_fu @event.photo, nil %>
```

We were seeing the following error:

```
Started GET "/system/images/W1siZiIsIjIwMTQvMTIvMTMvMDlfMzdfMDlfOTQxX2x1bWJlcmphY2sucG5nIl0sWyJwIiwidGh1bWIiLG51bGxdLFsicCIsInN0cmlwIl1d/lumberjack.png" for 127.0.0.1 at 2014-12-13 09:45:46 +1300

ArgumentError (Didn't recognise the geometry string ):
```

This is worked around by simply not supplying a geometry string when nil is requested as geometry, because we simply don't need to try to do any thumbnailing.