Fix sample presenter not properly determing if its empty (microsoft#1374)

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## Description
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The sample presenter did not properly determine if it was empty
resulting in it not hiding despite it being empty.
## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here. -->
Fixes microsoft#1373
## How Has This Been Tested?
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## Screenshots (if appropriate):

## Types of changes
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- [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)

Author: marcelwgn

WinUIGallery/Controls/SampleCodePresenter.xaml.cs

@@ -67,7 +67,7 @@ public IList<ControlExampleSubstitution> Substitutions
             set { SetValue(SubstitutionsProperty, value); }
         }
 
-        public bool IsEmpty => Code.Length == 0 && CodeSourceFile == null;
+        public bool IsEmpty => string.IsNullOrEmpty(Code) && string.IsNullOrEmpty(CodeSourceFile);
 
         private string actualCode = "";
         private static Regex SubstitutionPattern = new Regex(@"\$\(([^\)]+)\)");
@@ -88,7 +88,7 @@ private static void OnDependencyPropertyChanged(DependencyObject target, Depende
 
         private void ReevaluateVisibility()
         {
-            if (Code.Length == 0 && CodeSourceFile == null)
+            if (IsEmpty)
             {
                 Visibility = Visibility.Collapsed;
             }