[sgen-gray-queue] {WIP} Add null field on GrayQueueSection

This field is used to represent an empty section, it is equivalent to "end == 0".
[sgen-gray-queue] {WIP} Add SGEN_GRAY_QUEUE_ENTRY_SECTION which returns the section of the current entry
[sgen-gray-queue] {WIP} Fix enqueuing to an empty queue.
[sgen-gray-queue] {WIP} Fix dequeuing of an empty queue
[sgeb-gray-queue] {WIP} Remove unused SGEN_GRAY_QUEUE_ENTRY_NEXT
[sgen-gray-queue] {WIP} Move null object enqueuing assertion to general case
{WIP}