Do not introduce zero magmoms in SpacegroupAnalyzer

[lbluque]

Summary

Do not introduce zero magmoms in SpacegroupAnalyzer following changes in spglib >= 2. See #2725

TODO

• With this change, the exception tested in the unit test added in Raise ValueError in SpacegroupAnalyzer.get_symmetrized_structure() if spglib returns no symmetries #2724 is not raised.

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[coveralls]

Coverage remained the same at 0.0% when pulling 803d750 on lbluque:master into d17a0f4 on materialsproject:master.

[janosh]

This is great! Thanks @lbluque. Just tried out your changes on another 5 structures for which spglib with 0 magmons gave me no symmetries. This fix works for all of them!

I think it might still be useful to keep the test that raises ValueError on the Co8 structure I added in #2724. Just so we keep the nicer error message in case a similar problem reappears in the future. We can just manually insert magmoms = [0] * len(struct) in the test _cell to still make it raise the ValueError.