gh-41138: fix is_free method for new version Singular
Since the Singular 4.4.1p4 treat the zero element as the lastest element
of free resolution. So we need to check the lastest term is zero or not.
And someone may use the system's singular. Like the Ubuntu package is
4.4.1p3. It also has the problem.
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URL: https://github.com/sagemath/sage/pull/41138
Reported by: Chenxin Zhong
Reviewer(s): Dima Pasechnik
