Dropped NaN in eval_hermite

[jacksonvanover]

When the first argument is zero and the second argument is a NaN, eval_hermite returns 1.0. When the first argument is nonzero, the NaN is propagated as would be expected.

Reproducing code example:

Python 3.6.8 (default, Oct  7 2019, 12:59:55) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from scipy import special
>>> import numpy as np
>>> special.eval_hermite(0, np.nan)
1.0
>>> special.eval_hermite(1, np.nan)
nan

Scipy/Numpy/Python version information:

1.4.1 1.17.4 sys.version_info(major=3, minor=6, micro=8, releaselevel='final', serial=0)