fix: revert test changes

tests/Avalonia.Base.UnitTests/Media/TextFormatting/GraphemeBreakTestDataGenerator.cs

@@ -68,7 +68,7 @@ namespace Avalonia.Visuals.UnitTests.Media.TextFormatting
 
                         var graphemeChars = elements[0].Replace(" × ", " ").Split(' ');
 
-                        var codepoints = graphemeChars.Where(x => !string.IsNullOrEmpty(x) && x != "×")
+                        var codepoints = graphemeChars.Where(x => x != "" && x != "×")
                             .Select(x => Convert.ToInt32(x, 16)).ToList();
 
                         var grapheme = codepoints.ToArray();
@@ -77,7 +77,7 @@ namespace Avalonia.Visuals.UnitTests.Media.TextFormatting
                         {
                             var remainingChars = elements[1].Replace(" × ", " ").Split(' ');
 
-                            var remaining = remainingChars.Where(x => !string.IsNullOrEmpty(x) && x != "×").Select(x => Convert.ToInt32(x, 16)).ToArray();
+                            var remaining = remainingChars.Where(x => x != "" && x != "×").Select(x => Convert.ToInt32(x, 16)).ToArray();
 
                             codepoints.AddRange(remaining);
                         }

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -99,7 +99,7 @@ namespace Avalonia.Controls.UnitTests
 
                 textbox.Text = String.Empty;
                 Dispatcher.UIThread.RunJobs();
-                Assert.True(string.IsNullOrEmpty(control.SearchText));
+                Assert.True(control.SearchText == String.Empty);
                 Assert.False(control.IsDropDownOpen);
                 Assert.True(closeEvent);
             });

tests/Avalonia.Controls.UnitTests/MaskedTextBoxTests.cs

@@ -559,7 +559,7 @@ namespace Avalonia.Controls.UnitTests
                     Text = "0123456789"
                 };
 
-                Assert.True(string.IsNullOrEmpty(target.SelectedText));
+                Assert.True(target.SelectedText == "");
 
                 target.SelectionStart = 2;
                 target.SelectionEnd = 4;

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -473,7 +473,7 @@ namespace Avalonia.Controls.UnitTests
                 
                 target.ApplyTemplate();
 
-                Assert.True(string.IsNullOrEmpty(target.SelectedText));
+                Assert.True(target.SelectedText == "");
 
                 target.SelectionStart = 2;
                 target.SelectionEnd = 4;