WebContent: Return empty JSON object if element has no box model
The blank string "" does not parse as JSON, and so the InspectorWidget
would fail to update the box-model information when inspecting elements
with no box, (for example, `<head>`) showing stale values instead. Now,
they show all 0s.
You could argue that InspectorWidget should be more resilient when given
invalid JSON strings, but making sure we only pass valid ones works
too.
Sam Atkins