`divide by zero encountered in double_scalars` when converting units using `equivalencies`

Description

Converting wavelength to frequency using equivalencies returns a warning if the wavelength is zero.

/home/atharva/miniconda3/envs/astropy-test/lib/python3.9/site-packages/astropy/units/equivalencies.py:126: RuntimeWarning: divide by zero encountered in double_scalars
  (si.m, si.Hz, lambda x: _si.c.value / x),

Expected behavior

The code returns inf Hz, but with the warning, which it shouldn’t.

Steps to Reproduce

import astropy.units as u
a = 0* u.AA
a = a.to(u.Hz, equivalencies=u.spectral())
print(a)

Returns:

/home/atharva/miniconda3/envs/astropy-test/lib/python3.9/site-packages/astropy/units/equivalencies.py:126: RuntimeWarning: divide by zero encountered in double_scalars
  (si.m, si.Hz, lambda x: _si.c.value / x),
inf Hz

Any other value apart from zero doesn’t cause the warning.

import astropy.units as u
import math
a = math.inf* u.AA
a = a.to(u.Hz, equivalencies=u.spectral())
print(a) # 0.0 Hz

System Details

Linux-5.8.0-55-generic-x86_64-with-glibc2.29
Python 3.8.10 (default, Jun  2 2021, 10:49:15) 
[GCC 9.4.0]
Numpy 1.21.2
astropy 5.0.dev828+g23404eeb2

[mhvk]

@pllim is right - we’re just propagating the numpy warning here, and I think that is probably the best thing to do, since it is quite likely to be a bug if the wavelength is zero.

That is not to say one could not have decided otherwise, and it is always good to think about these issues, so still good to report if something does not feel logical!

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