Update instructions.md

[mattshu]

original instructions made it seem extra step was needed (signal an error rather than return empty string). also updated expected output

[wolf99]

Hi @mattshu
Thank you for making a commit to improve exercism!

However, these instructions are synced for all languages tracks from a central source.
If you would like to change these it is best to make the change there: https://github.com/exercism/problem-specifications