Pywal crashes in empty directory with iterative argument

[MTBorg]

Running pywal with the iterative argument in an empty directory, .e.g.

wal -i <empty directory> --iterative

produces the stacktrace

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/pywal/image.py", line 54, in get_next_image
    image = images[next_index]
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/wal", line 11, in <module>
    load_entry_point('pywal==3.3.0', 'console_scripts', 'wal')()
  File "/usr/lib/python3.8/site-packages/pywal/__main__.py", line 208, in main
    parse_args(parser)
  File "/usr/lib/python3.8/site-packages/pywal/__main__.py", line 162, in parse_args
    image_file = image.get(args.i, iterative=args.iterative)
  File "/usr/lib/python3.8/site-packages/pywal/image.py", line 69, in get
    wal_img = get_next_image(img)
  File "/usr/lib/python3.8/site-packages/pywal/image.py", line 57, in get_next_image
    image = images[0]
IndexError: list index out of range

It does not happen when the --iterative flag isn't used.
The error does not really matter as it has no effect, but a clean exit should be prefered.

[MTBorg]

Got a fix in #505.