fix(tags): show no tooltip on a tag badge without a description

The badge bound its tooltip to the description directly, so a tag with an
empty description popped up an empty tooltip.

YKanBan.Tests/ViewModels/BoardViewModelTests.cs

@@ -456,6 +456,13 @@ public class BoardViewModelTests
         Assert.AreEqual(1, fixture.Repository.GetTags().Count);
     }
 
+    [TestMethod]
+    public void TagBadgeShowsNoTooltipForAnEmptyDescription()
+    {
+        Assert.IsNull(new TagBadgeViewModel(new TagModel(1, "bug", new RgbColor(1, 2, 3), "")).ToolTip);
+        Assert.AreEqual("why", new TagBadgeViewModel(new TagModel(1, "bug", new RgbColor(1, 2, 3), "why")).ToolTip);
+    }
+
     [TestMethod]
     public void TagPickerHidesAssignedTagsAndFiltersCaseInsensitively()
     {

YKanBan/ViewModels/Board/TagBadgeViewModel.cs

@@ -37,6 +37,12 @@ public sealed class TagBadgeViewModel : ViewModelBase
     /// </summary>
     public string Description => Tag.Description;
 
+    /// <summary>
+    /// Gets the badge tooltip: the description, or <see langword="null"/> (no
+    /// tooltip at all) when the description is empty.
+    /// </summary>
+    public string? ToolTip => Tag.Description.Length > 0 ? Tag.Description : null;
+
     /// <summary>
     /// Gets the badge fill: the tag's own color.
     /// </summary>

YKanBan/Views/Dialogs/CardEditorDialog.axaml

@@ -45,7 +45,7 @@
                 <ItemsControl.ItemTemplate>
                     <DataTemplate x:DataType="board:TagBadgeViewModel">
                         <Border Background="{Binding Background}" CornerRadius="10" Padding="8,2,4,2"
-                                Margin="0,0,6,6" ToolTip.Tip="{Binding Description}">
+                                Margin="0,0,6,6" ToolTip.Tip="{Binding ToolTip}">
                             <StackPanel Orientation="Horizontal" Spacing="2">
                                 <TextBlock Text="{Binding Name}" Foreground="{Binding Foreground}"
                                            VerticalAlignment="Center" FontSize="12"/>