Concatenate `conditions` with `operator` and wrap it by ().

    It returns a string in a list or empty list, if `conditions` is empty.

def concat_expr(operator, conditions):
    """
    Concatenate `conditions` with `operator` and wrap it by ().

    It returns a string in a list or empty list, if `conditions` is empty.

    """
    expr = " {0} ".format(operator).join(conditions)
    return ["({0})".format(expr)] if expr else []