Clarify that the v3 Webhook event.agent can have a null

[metavida]

Description

Before this change v3 Webhook Overview docs already implied that the event.agent can have a null value because it's demonstrated in the "example webhook payload for a service.updated event is shown below."

This PR makes the Webhook Payload documentation more clear via the following changes:

1. Specifies that the event.agent object is specifically a Resource Reference.
2. Explicitly states that the event.agent object can have a null value.
3. Explicitly states that the event.client object can have a null value.

It's worth nothing that I have not done a comprehensive analysis of whether other Webhook event fields can also have null values.

Jira Ticket

• https://pagerduty.atlassian.net/browse/ORCA-5086

Before Merging!

• Check staging environment to ensure changes look as intended.
  
• Ensure there is a review from SHAF Shared and from Community.