FutureWarning: split() requires a non-empty pattern match

[pllim]

I found this while investigating astropy/astropy#7928 by:

• Un-silencing warnings in pytest.
• Turn warnings into exception (except for ImportWarning from Cython, which I ignored still).
• Run tests on coordinates (saw this in convolution too).

__________________ [doctest] docs/coordinates/solarsystem.rst __________________
.../pytest_doctestplus/plugin.py:122: in runtest
    raise_on_error=True, verbose=False, encoding='utf-8')
.../doctest.py:2073: in testfile
    test = parser.get_doctest(text, globs, name, filename, 0)
.../doctest.py:669: in get_doctest
    return DocTest(self.get_examples(string, name), globs,
.../doctest.py:683: in get_examples
    return [x for x in self.parse(string, name)
.../pytest_doctestplus/plugin.py:192: in parse
    required = re.split(r'\s*,?\s*', match.group(1))
.../re.py:212: in split
    return _compile(pattern, flags).split(string, maxsplit)
E   FutureWarning: split() requires a non-empty pattern match.

https://github.com/astropy/pytest-doctestplus/blob/6703a182cf931968b5a9f2ac1949059270a88002/pytest_doctestplus/plugin.py#L213-L214

Here is my pytest header:

platform linux -- Python 3.6.4, pytest-3.2.1, py-1.4.34, pluggy-0.4.0

Running tests with Astropy version 3.1.dev22915.
Running tests in astropy/coordinates docs/coordinates.

Date: 2018-10-26T15:57:40

Platform: Linux-3.10.0-862.14.4.el7.x86_64-x86_64-with-redhat-7.5-Maipo

Executable: .../envs/py36/bin/python

Full Python Version: 
3.6.4 |Anaconda, Inc.| (default, Jan 16 2018, 18:10:19) 
[GCC 7.2.0]

encodings: sys: utf-8, locale: UTF-8, filesystem: utf-8
byteorder: little
float info: dig: 15, mant_dig: 15

Numpy: 1.15.2
Scipy: 1.1.0
Matplotlib: 2.2.2
h5py: 2.8.0
Pandas: 0.20.3
Asdf: 2.0.1
Cython: 0.26.1
astropy_helpers: 2.0.6
Using Astropy options: remote_data: any.

PyQt5 5.6 -- Qt runtime 5.6.2 -- Qt compiled 5.6.2
rootdir: /tmp/astropy-test-woysnyxx/lib/python3.6/site-packages, inifile: setup.cfg
plugins: xdist-1.17.1, qt-2.4.1, openfiles-0.2.0, mpl-0.8, flake8-0.8.1, doctestplus-0.1.2, arraydiff-0.1, remotedata-0.2.2.dev0, ci-watson-0.0.0.dev33+gdec4b009