A Python function to identify the first unique character in a string, case-insensitively.
The task is to design a Python function named `first_unique_char()`, which identifies the first character in a provided string `s` that appears only once. This function should not differentiate between uppercase and lowercase characters, i.e., it should consider them to be the same. ### Parameters - `s` (str): This is the input string for which the function will identify the first unique character. ### Return Value - If there is a character in `s` that appears only once, the function should return this character as it was originally entered (uppercase or lowercase). It should not transform the case prior to returning the character. - If no such character exists, the function should return an empty string. ### Examples ```python # 'e' appears once and is the first to do so first_unique_char('moonmen') # Returns 'e' # 't' appears once and is the first to do so first_unique_char('sTreSS') # Returns 't' ```