ServiceUrl should not be manditory for expect replies in delivery mode

[MattB-msft]

Describe the bug
When sending an activity with ExpectReplies in the delivery mode property. ServiceURL should not be required.

To Reproduce
Send an activity with deliveryMode = ExpectReplie without populating the Service URL
an exception is raised indicating that the ServiceUrl is required.

Expected behavior
Send an activity with deliveryMode = ExpectReplie without populating the Service URL
Request should be honored without error