Fever API - Fix Mark all as read for feed unread

Fixes #255

Zero is a valid super group id according to the "mark the Kindling super group
as read" example in the Fever API doc. But the php function empty considers 0
as an empty value and the condition is never true.

The condition has been changed to accept -1 for the "Sparks" super group,
0 for the "Kindling" super group and any other positive integer (feed and
item id must be positive integers).

The group id Zero is used by Reeder for iOS for the virtual feed "unread".