Make Event::getConversationUuid() return type nullable

[erickskrauch]

Description

We spotted a webhook with null in the conversation_uuid field.

{
    "conversation_uuid": null,
    "direction": "inbound",
    "duration": "0",
    "from": "*filtered*",
    "headers": [],
    "network": "20822",
    "price": "0.00000000",
    "rate": "0.00820000",
    "start_time": null,
    "status": "completed",
    "timestamp": "2024-02-12T13:25:35.241Z",
    "to": "*filtered*",
    "uuid": "835b549816baf1f8ce18e1088fb1b95a"
}

Motivation and Context

We need to fix the type error.

How Has This Been Tested?

In production.

Example Output or Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8cc9df0) 79.18% compared to head (46bccbc) 79.18%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #468 +/- ## ========================================= Coverage 79.18% 79.18% Complexity 2371 2371 ========================================= Files 218 218 Lines 6356 6356 ========================================= Hits 5033 5033 Misses 1323 1323 ```

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