Support validating UUIDs including the nil UUID

[sleberknight]

While UUIDs#isValidUUID does not support the nil UUID, we should add a variant that does consider nil UUIDs be be valid.

For example:

    private static final String NIL_UUID = "00000000-0000-0000-0000-000000000000";

   // ...

    public static boolean isValidUUIDAllowingNil(String value) {
        return NIL_UUID.equals(value) || isValidUUID(value);
    }