gh-128923: Use zero to indicate unassigned unique id (#128925)

gh-128923: Use zero to indicate unassigned unique id (#128925)

In the free threading build, the per thread reference counting uses a
unique id for some objects to index into the local reference count
table. Use 0 instead of -1 to indicate that the id is not assigned. This
avoids bugs where zero-initialized heap type objects look like they have
a unique id assigned.