def common_prefix(strings=None):
    """ Find the longest string that is a prefix of all the strings.

    >>> common_prefix(['hello_bob','hello_jane','hello_dan'])
    'hello_'

    >>> common_prefix(['joe','george'])
    ''

    >>> common_prefix(['joe','jerry'])
    'j'

    >>> common_prefix([])
    ''

    """
    if strings is None:
        return ''
    if len(strings) == 0:
        return ''
    prefix = strings[0]
    for s in strings:
        if len(s) < len(prefix):
            prefix = prefix[:len(s)]
        if not prefix:
            return ''
        for i in range(len(prefix)):
            if prefix[i] != s[i]:
                prefix = prefix[:i]
                break
    return prefix
