Don't return hover when there's no documentation

[fbricon]

Empty hover with only link to source looks weird.

Might as well not return anything

[angelozerr]

Source information can be interesting I think, no?

[fbricon]

I feel that adds unnecessary bloat. I'd say hide it if no visible documentation to display. If someone requests we display links to sources when hovering, even without doc, then we can add a new preference.