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UTC->UT1 conversion fails for zero-length Time object

See original GitHub issue

Description

Converting a zero-length Time object from UTC to UT1 fails because the IERS _interpolate method does not handle the corner case of an empty array.

Example code:

from astropy.time import Time
from astropy import units as u
import numpy as np

time = Time.now() + np.arange(24) * u.hour

indices = np.asarray([0, 1, 2], dtype=np.intp)
print(time[indices].ut1) # This is fine

empty_indices = np.asarray([], dtype=np.intp)
print(time[empty_indices].ut1)  # KABOOM

Expected behavior

Should print:

[datetime.datetime(2021, 3, 31, 18, 17, 6, 727233)
 datetime.datetime(2021, 3, 31, 19, 17, 6, 727213)
 datetime.datetime(2021, 3, 31, 20, 17, 6, 727193)]
[]

Actual behavior

Actually prints:

[datetime.datetime(2021, 3, 31, 18, 17, 6, 727233)
 datetime.datetime(2021, 3, 31, 19, 17, 6, 727213)
 datetime.datetime(2021, 3, 31, 20, 17, 6, 727193)]
Traceback (most recent call last):
  File "./test.py", line 32, in <module>
    print(time[empty_indices].ut1)  # KABOOM
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/time/core.py", line 1327, in __getattr__
    tm._set_scale(attr)
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/time/core.py", line 576, in _set_scale
    args.append(get_dt(jd1, jd2))
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/time/core.py", line 1944, in _get_delta_ut1_utc
    delta = iers_table.ut1_utc(jd1, jd2)
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/utils/iers/iers.py", line 276, in ut1_utc
    return self._interpolate(jd1, jd2, ['UT1_UTC'],
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/utils/iers/iers.py", line 360, in _interpolate
    self._refresh_table_as_needed(mjd)
  File "/private/tmp/env/lib/python3.8/site-packages/astropy/utils/iers/iers.py", line 719, in _refresh_table_as_needed
    max_input_mjd = np.max(mjd)
  File "<__array_function__ internals>", line 5, in amax
  File "/private/tmp/env/lib/python3.8/site-packages/numpy/core/fromnumeric.py", line 2733, in amax
    return _wrapreduction(a, np.maximum, 'max', axis, None, out,
  File "/private/tmp/env/lib/python3.8/site-packages/numpy/core/fromnumeric.py", line 87, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
ValueError: zero-size array to reduction operation maximum which has no identity

Steps to Reproduce

(See sample code above)

System Details

>>> import platform; print(platform.platform())
macOS-10.15.7-x86_64-i386-64bit
>>> import sys; print("Python", sys.version)
Python 3.8.8 (default, Mar 18 2021, 06:01:57) 
[Clang 11.0.3 (clang-1103.0.32.62)]
>>> import numpy; print("Numpy", numpy.__version__)
Numpy 1.20.2
>>> import astropy; print("astropy", astropy.__version__)
astropy 4.1.dev3458+g7811614f8
>>> import scipy; print("Scipy", scipy.__version__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'scipy'
>>> import matplotlib; print("Matplotlib", matplotlib.__version__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'matplotlib'