[Bug] DataTrigger not applied when null at initial

### Description

The `DataTrigger` is not applied if the value is `null` at the beginning, but it is applied later if the value changes.

Binding to:
- **Nullable** type with default value => Trigger **not applied** at beginning
- **Non nullable** type with default value => Trigger **applied** at beginning

So the trigger is applied at the beginning only if the Binding value is not null. I think the behavior is not consistent, the trigger should be applied at the beginning as well, even if the value is null.

Reproduction project available here: https://github.com/MADSENSE/MADSENSE.MAUI.Sample.App/tree/datatrigger-initial-null

Already tracked by https://github.com/xamarin/Xamarin.Forms/issues/13544

### Steps to Reproduce

1. Define DataTrigger with Binding to nullable type
2. The DataTrigger is not applied at beginning, only on value change back to null

### Expected Behavior
`DataTrigger` applied at beginning, even if null binding value.

### Actual Behavior
`DataTrigger` with null binding value not applied at beginning.
BTW it seems that the DataTrigger keeps the last value (eg: on Value=2, the `BackgroundColor` should revert back to transparent)

### Version with bug

Release Candidate 2 (current)

### Last version that worked well

Unknown/Other

### Affected platforms

iOS, Android, Windows

### Affected platform versions

Windows 10.0.19041

### Did you find any workaround?

Define `Binding.TargetNullValue` or `Binding.Converter`

### Relevant log output

_No response_

Start with the linked MADSENSE.MAUI.Sample.App reproduction on its datatrigger-initial-null branch and run the nullable-binding case on an affected platform. Compare the initial null state with a later change back to null; done means the DataTrigger applies at startup and the property reverts correctly when the binding is null.

Written by the indexing model from the issue text.