Updated tests which expected SetValue(key, null) to maintain record

Previously, calling `SetValue(key, null)` would set the `TrackedRecord<T>.Value` to null, but wouldn't delete the record. This was confusing, if not buggy, and the behavior was fixed (3d1ad6b, 0bccf1f). In doing so, however, unit tests which validated the previous assumption broke, and needed to be updated.

This isn't expected to harm any actual implementations, as they should already be written to account for missing records, since this is the default when loading from the database. The null value scenario would only occur when updating previously saved values.

Author: JeremyCaney

OnTopic.Tests/TopicReferenceCollectionTest.cs

@@ -220,7 +220,7 @@ public void SetValue_ExistingReference_TopicUpdated() {
     ///   TrackedRecordCollection{TItem, TValue, TAttribute}.SetValue(String, TValue, Boolean?, DateTime?)"/>, updates the
     ///   reference using <see cref="TrackedRecordCollection{TItem, TValue, TAttribute}.SetValue(String, TValue, Boolean?,
     ///   DateTime?)"/> with a <c>null</c> value, and confirms that the <see cref="Topic"/> reference and <see cref="Topic.
-    ///   IncomingRelationships"/> are correctly updated.
+    ///   IncomingRelationships"/> are correctly removed.
     /// </summary>
     /// <remarks>
     ///   This calls <see cref="TrackedRecordCollection{TItem, TValue, TAttribute}.SetValue(String, TValue, Boolean?, DateTime?)
@@ -238,7 +238,7 @@ public void SetValue_ExistingReference_IncomingRelationshipsUpdates() {
       topic.References.SetValue("Reference", null);
       topic.References.SetValue("Reference", null);
 
-      Assert.True(topic.References.Contains("Reference"));
+      Assert.False(topic.References.Contains("Reference"));
       Assert.Null(topic.References.GetValue("Reference"));
       Assert.Equal<int?>(0, reference.IncomingRelationships.GetValues("Reference")?.Count);
 
@@ -262,7 +262,7 @@ public void SetValue_NullReference_TopicRemoved() {
       topic.References.SetValue("Reference", reference);
       topic.References.SetValue("Reference", null);
 
-      Assert.Single(topic.References);
+      Assert.Empty(topic.References);
       Assert.Null(topic.References.GetValue("Reference"));
 
     }