uci:get_list: always return a table, no matter what.

uci:get_list: always return a table, no matter what.

Previously, get_list("fake", "non-existent", "notreal") would still
return a table, just empty.  This is nice, as you can always iterate the
returned table, without having to check it first.

However, if you happened to pass a nil for any of the parameters, you
would actually get a nil in return.  This was inconsistent.

The documentation is updated to clarify the behaviour of this function.

Signed-off-by: Karl Palsson <karlp@remake.is>