Add test for zero-length case on exercise "series"

I noticed that the exercise “series” was missing a test for the zero-length case that is described in this paragraph:

Different languages on Exercism have different expectations about what the result should be if the length of the substrings is zero.
For Rust, we expect you to output a number of empty strings, which will be one greater than the length of the input string.

I made a PR to add that test, but the auto-responder directed me to this forum. Please let me know what you think.

Edit: I found the forum post talking about dropping that test. But in that case, I think the readme should be amended to match that test being removed, shouldn’t it?

You’re absolutely right. I will fix the documentation shortly. Thanks for the report!

link to PR for reference