GraphPresenter doesn't work in a UWP app inside a Uno project

## Describe the bug
The GraphPresenter control works fine when it's executed in a standalone UWP app, but it doesn't render when the same UWP app is part of an Uno-based solution.

- [ ] Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

## Steps to Reproduce
Steps to reproduce the behavior:
1. Create a new Uno solution
2. Add the Microsoft.Toolkit.Graph.Controls NuGet to the UWP project
3. Add the following XAML to the MainPage.xaml file in the shared project:

```

<Setter Property="Margin" Value="0,8,0,8"/>

```
4. Run the UWP app

## Expected behavior
A list of mail, presented through the GraphPresenter control, is displayed in the main page. The control, instead, is empty.
The same exact XAML code in a standalone UWP app (not part of an Uno solution) works fine.

## Screenshots
This is the output of the UWP app included in the Uno solution:


This is the output, instead, of a standalone UWP app:


## Environment

```
NuGet Package(s): Microsoft.Toolkit.Graph.Controls

Package Version(s): 7.0.0-preview2

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] October 2020 Update (19042)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] October 2020 Update (19042)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio
- [ ] 2017 (version: )
- [x] 2019 (version: 16.7.7 )
- [ ] 2019 Preview (version: )

```

## Additional context
Add any other context about the problem here.