Solve a coding problem involving Python basics like constructor, pass, string operations, and finding the first most non-duplicate element in a list.
Implement first_non_duplicate(items) for a list of strings. Return the first element whose complete string value occurs exactly once, preserving list order. Return an empty string when every element is repeated or the list is empty. Matching is case-sensitive, and whitespace is part of each string.
def first_non_duplicate(items):