Avoid building node GRES data for count of zero

Previously with a configuration including FastSchedule=1 or 2,
GRES with files defined in gres.conf, and in the node configuration
either specified with a count of 0 or not included at all then
the node's GRES data would be built with a topology array containing
a GRES bitmap of zero size, resulting in an abort when trying to
operate on that bitmap.

bug 6997