python: provide informative exception for trailing escapes in tables

[kieran-ryan]

🤔 What's changed?

With the Python implementation:

1. Whitespace either side of a table row is stripped.
2. The table row is passed to a function to strip and yields its cell values
3. Characters in each cell are iterated over to handle them appropriately, such as new columns (|), escape characters, etc.
4. As escape characters are typically followed by a special character (e.g. n, t), when an escape character is detected \\ an additional next call is made to grab the next character
5. As in step 1, whitespace after a trailing escape character (\\) would be stripped, there would not be a following character and a StopIteration error is raised.

https://github.com/cucumber/gherkin/blob/b055261c59f5f905fe637b230017054250c6ebb5/python/gherkin/gherkin_line.py#L39-L62

By defaulting the iterator to an empty string, when there is a trailing escape character, the trailing escape character will not trigger a StopIteration error and subsequent code will appropriately raise an inconsistent cell count error message.

⚡️ What's your motivation?

• Fixes #223

🏷️ What kind of change is this?

• :bug: Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

📋 Checklist:

• [x] I agree to respect and uphold the Cucumber Community Code of Conduct
• [x] I've changed the behaviour of the code
  • [x] I have added/updated tests to cover my changes.
• [x] Users should know about my change
  • [x] I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[mpkorstanje]

Not a problem in JS, while we do append undefined, this cell is never used because it is not terminated by a |.