Render the empty tag expression as an empty string

[mpkorstanje]

🤔 What's changed?

While the empty tag expression, results in a True expression. This
True expression should be rendered as the empty string to make it
possible to parse the string representation of a parsed expression.

⚡️ What's your motivation?

Fixes: #195

🏷️ What kind of change is this?

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

📋 Checklist:

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