def conf_to_xml(conf):
    """
    Converts LensConf given as dictionary to xml string
    :param conf: a Dictionary
    :return: LensConf xml string representation
    >>> conf_to_xml(None)
    '<conf></conf>'
    >>> conf_to_xml({})
    '<conf></conf>'
    >>> conf_to_xml({'a':'b'})
    '<conf><properties><entry><key>a</key><value>b</value></entry></properties></conf>'
    """
    if conf and len(conf) != 0:
        return "<conf><properties>" + \
               "".join("<entry><key>%s</key><value>%s</value></entry>" % (k, v) for k, v in conf.items()) + \
               "</properties></conf>"
    return "<conf></conf>"