Clean up Build Boss code style and error messaging

src/Tools/BuildBoss/InternalsVisibleTo.cs

@@ -21,13 +21,24 @@ public override string ToString()
         {
             var element = new XElement("InternalsVisibleTo");
             if (TargetAssembly is object)
+            {
                 element.Add(new XAttribute("Include", TargetAssembly));
+            }
+
             if (PublicKey is object)
+            {
                 element.Add(new XAttribute("Key", PublicKey));
+            }
+
             if (LoadsWithinVisualStudio is object)
+            {
                 element.Add(new XAttribute("LoadsWithinVisualStudio", LoadsWithinVisualStudio));
+            }
+
             if (WorkItem is object)
+            {
                 element.Add(new XAttribute("WorkItem", WorkItem));
+            }
 
             return element.ToString();
         }

src/Tools/BuildBoss/ProjectCheckerUtil.cs

@@ -255,8 +255,14 @@ private bool CheckInternalsVisibleTo(TextWriter textWriter)
                     continue;
                 }
 
-                if (Uri.TryCreate(internalsVisibleTo.WorkItem, UriKind.Absolute, out _))
+                if (!string.IsNullOrEmpty(internalsVisibleTo.WorkItem))
                 {
+                    if (!Uri.TryCreate(internalsVisibleTo.WorkItem, UriKind.Absolute, out _))
+                    {
+                        textWriter.WriteLine($"InternalsVisibleTo for external assembly '{internalsVisibleTo.TargetAssembly}' does not have a valid URI specified for {nameof(InternalsVisibleTo.WorkItem)}.");
+                        allGood = false;
+                    }
+
                     // A work item is tracking elimination of this IVT
                     continue;
                 }