Null is a specific thing - the variable is either null or it's a string, not both. Empty strings ("") are sometimes called null-string. http://stackoverflow.com/a/5615766/2066278

"Null-Strings should be treated as if they are empty strings." ... null strings are empty - maybe you meant "Null should be treated as a null-string" or "Null and null-strings should be treated the same way"?

The test message about proto "methot" has typos and is unhelpful. Really the kata should say up front if it wants you to avoid a technique and not comment on what is or is not best practice (that's left up the voting system post submission).