luci-app-olsr: handle empty result for non-status tables

The response of the jsoninfo request may be non-empty (e.g. a linebreak
or just whitespace), but still yield "nil" after "json.decode".
This situation was already handled for the "status" request, but it
leads to a dictionary access against a "nil" value for all other tables.

Signed-off-by: Lars Kruse <lists@sumpfralle.de>