Fix selective_dyn handling issue in python>=3.12

[kavanase]

Hi,
I noticed I was getting some deprecation warnings like these, when parsing vasprun.xml files with python=3.12:

[DeprecationWarning("Testing an element's truth value will always return True in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead."), DeprecationWarning("Testing an element's truth value will always return True in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead.")]

I traced this down to this line (https://github.com/materialsproject/pymatgen/blob/master/src/pymatgen/io/vasp/outputs.py#L1465).
With this small update, it fixes selective_dyn handling (as mentioned in the warning, this would always be set to True with future versions) and avoids these deprecation warnings.

[shyuep]

Thanks.