Workaround for instance creation (empty object URIs)

This is a workaround for a problem that may occur when trying to create a new instance at "/resource/instances" via "Add Instance" button.

The error is originated by a failing AJAX request that tries to save the instance:
![add_instance_error](https://cloud.githubusercontent.com/assets/1383323/2601269/1d993d86-bb0d-11e3-818c-2c590ea59cd6.png)
The request contains statements with empty object URIs, which cannot be processed by Virtuoso.

This workaround checks the given statements and removes those with empty object URIs.

The underlying issue seems to be related to line 467 in the JavaScript file extensions/themes/silverblue/scripts/support.js, which clears the object URIs.
However, I have no idea about the intention of that code and I do not know how to fix the editor. For the time being it seems reasonable to include at least a workaround.