Check if a uri callback actually returns an uri

In the funciton entity_uri, there is the possibility to create an URI by a callback function.
But after calling that callback, there is no check if it actually returned something.

As an example: the entity module has a default entity uri callback to allow other modules to create a uri by a hook.
But when no hooks are present, the callback doesn't return a uri. In my opinion a legal result.

But the entity_uri function assumes the callback always returns a value, and appends some options.
Because of this, a invalid response is returned, because the 'path' property is missing.

I will attach a patch.