GVariant: don't remove padding from no field

GVariant: don't remove padding from no field

When a message is created with dbus_message_new() it gets no fields.
Then, while appending some values, but before setting path, interface,
etc. a single field appears - signature. If there would be more fields,
then after deleting the last field we would need to remove padding
from second-to-last field. In case of a single field, we have
no second-to-last field, only empty dictionary. Empty dictionaries do
not have padding at the end, so we do not need to remove it.

Change-Id: Idb15f3999a3ff0117f906ec632f462eb5cdaab09