Ensure presenter visibility in sample rendering (#2050)

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## Description
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This PR ensures that the code presenter inside the `SampleCodePrenseter`
custom control is visible whenever it has code to render.

## Motivation and Context
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Fixes #2049.

## How Has This Been Tested?
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Manually tested.

## Screenshots (if appropriate):

## Types of changes
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the boxes that apply: -->
- [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)

Co-authored-by: Niels Laute <[email protected]>

Author: AndrewKeepCoding

WinUIGallery/Controls/SampleCodePresenter.xaml.cs

@@ -194,6 +194,9 @@ private async void FormatAndRenderSampleFromFile(string sourceRelativePath, Cont
 
     private void FormatAndRenderSampleFromString(string sampleString, ContentPresenter presenter, ILanguage highlightLanguage)
     {
+        // Make sure the presenter is visible in case it was collapsed due to no code being present.
+        presenter.Visibility = Visibility.Visible;
+
         // Trim out stray blank lines at start and end.
         sampleString = sampleString.TrimStart('\n').TrimEnd();