BUG: printing Angle, Latitude, Longitude with a NaN in degrees

Description

Though nan degrees is a valid initializer, it cannot be printed

print(coord.Longitude(np.nan * u.deg))

Expected behavior

The same as nan radians, but in degrees.

print(coord.Longitude(np.nan * u.rad))

Actual behavior

degrees, unlike radians, appears to call sexagesimal_to_string which has int(values[1]), but nan cannot be converted to an integer, so it fails.

~/local/astropy/astropy/coordinates/angle_utilities.py in sexagesimal_to_string(values, precision, pad, sep, fields)
    637     literal = ''.join(literal)
    638     return literal.format(np.copysign(values[0], sign),
--> 639                           int(values[1]), values[2],
    640                           sep=sep, pad=pad,
    641                           last_value=last_value)

ValueError: cannot convert float NaN to integer

Steps to Reproduce

import astropy.units as u
import astropy.coordinates as coord
print(coord.Longitude(np.nan * u.deg))

System Details

macOS-10.16-x86_64-i386-64bit Python 3.8.2 | packaged by conda-forge | (default, Apr 24 2020, 07:56:27) [Clang 9.0.1 ] Numpy 1.18.4 astropy 4.3.dev721+g3646d9e6e.d20210403 Scipy 1.4.1 Matplotlib 3.4.1

[JasonS09]

Sure. I’ll take this issue then!

[mhvk]

To clarify since in #11491 there seems to be confusion, the issue here is not just that nan raises an error, but also that the output should make clear that the value is actually nan, i.e., it should be something like what happens for Quantity

In [3]: u.Quantity(np.nan, u.deg)
Out[3]: <Quantity nan deg>

Of course, a mix of valid and nan values should give sensible output as well.