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()
     {