When starting 10.2 with server-id=0 or without server-id configured, following note is printed:

2017-05-01 8:26:22 140643309708800 [Warning] option 'server_id': unsigned value 0 adjusted to 1

And the value indeed becomes '1'

MariaDB [(none)]> show variables like 'server_id';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| server_id     | 1     |

+---------------+-------+

According to documentation '0' is valid value for server_id indicating that this server is not intended to be part of replication.

Thus either 0 should preserve as valid value or documentation must be changed.