Revert "remove log.is_zero routine"

This reverts commit de47eb297f822edea3b70b8d8cf094072833b806.
Cleanup log._eval_is_zero, add tests
Drop trivial log(1).is_zero test [skip ci]
Merge pull request #8053 from skirpichev/restore-log-is_zero

Revert "remove log.is_zero routine"