String transformation benchmark: handle empty strings

[pwochner]

Make sure that empty strings are handled properly:

• Transformations
  • makeUppercase, makeLowercase, drop are not meaningful on an empty string and we don't want to encourage programs to do that -> throw error.
  • moveRight, moveLeft should return StringState unchanged.
• We want to be able to get from a non-empty string to an empty string by applying drop, but then also be able to add characters to it again.
• Conditions (atEnd, isLetter, etc.): Return false? Or error?