Mixpanel integration: falsy variant values collapse into enabled-state

### How are you running Flagsmith

- [ ] Self Hosted with Docker
- [ ] Self Hosted with Kubernetes
- [x] SaaS at flagsmith.com
- [ ] Some other way (add details in description below)

### Describe the bug

When a flag is enabled and the variant value is falsy, the mixpanel integration incorrectly fallbacks the value to the flag state `true` instead of `false`.

The documentation is ambiguous, only stating `if the flag has no remote config value, pass the boolean state`
```
# Current mapping

Flag state | Variant value | Sent to Mixpanel
-----------+----------------------------+----------------------
ON | True | true (value)
ON | False / None / 0 / "" | true (enabled state)
ON | "hello" / 42 | the value
OFF | anything | false (enabled state)
```

### Steps To Reproduce

1. Activate Mixpanel integration in your environment
2. Create a multivariate feature
3. Enable the flag
4. Set a variant value to be `false`
5. Mixpanel will show the variant as `true`

### Expected behavior

The mapping should be:
```
Flag state | Variant value | Sent to Mixpanel
-----------+----------------------------+----------------------
ON | True | true (value)
ON | False / "" / 0 | false
ON | None / null | true (enabled state)
ON | "hello" / 42 | the value
OFF | anything | false (enabled state)
```

The resolution of the issue includes:
- Updating the doc to be exhaustive in relation to the behavior
- Communicate with impacted customers before releasing

### Screenshots

_No response_