fix(nupdown): only set two_fermi when nupdown is non-zero

[dyzheng]

When nupdown=0.0 is explicitly set in INPUT, it should behave the same as not setting nupdown at all. Previously, reading any nupdown value would unconditionally set two_fermi=true, causing different code paths even when nupdown=0.0.

Now two_fermi is only set to true when nupdown != 0.0, making nupdown=0 equivalent to not specifying the parameter.

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Linked Issue

Fix #7299

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