sign of complex 0+0j returns complex nan

[WarrenWeckesser]

Example:

In [3]: import array_api_strict as aa

In [4]: z = aa.zeros(1, dtype=aa.complex128)

In [5]: aa.sign(z)
/home/warren/py3.13.5/lib/python3.13/site-packages/array_api_strict/_array_object.py:970: RuntimeWarning: invalid value encountered in divide
  res = self._array.__truediv__(other._array)
Out[5]: Array([nan+nanj], dtype=array_api_strict.complex128)

The result should be Array([0+0j], ...).

Special handling of the complex case was added in #74, but that change didn't consider the input 0+0j.