[lib] Allow calling endpoints with empty payload

Summary: At this point socket validation fails for requests without a payload. A new endpoint that was introduced in D7726 doesn't have a payload and we should allow sending a request to it.

Test Plan: Check if sending a request passes validation.

Reviewers: bartek, kamil, inka

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7727