<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def join(strings: Optional[Sequence[str]], separator: str = "") -> str: """Join strings in a given sequence. Return an empty string if it is None or empty, otherwise join all items together separated by separator if provided. """

    return separator.join(s for s in strings if s) if strings else ""