fix: handle blank push notification config id consistently across transports

[malladinagarjuna2]

GetTaskPushNotificationConfigParams rejects a null id but permits an
empty string, and the two transports diverge on that input:

• GrpcTransport sends .setId("").
• RestTransport treats it as absent and requests the trailing-slash
  "default config" URL.

Identical caller code against the same ClientTransport method therefore
produces two different requests. This aligns both on
and adds coverage for the blank-id path, which was previously untested.

Also corrects the Javadoc on GetTaskPushNotificationConfigParams, which
described id as optional and documented @throws for taskId or tenant
when the constructor actually validates taskId and id.