Introduced test to validate deleting existing attribute

If a previous attribute is deleted, the database is expected to return a `null` value for that record. If the topic already exists in the `referenceTopic` with a valid attribute for that `attributeKey`, then that attribute should then be deleted. This test confirms that is true.

(In fact, this test fails—but that's due to identifying an underlying bug. That bug will be fixed in a subsequent commit.)

Author: JeremyCaney

OnTopic.Tests/SqlTopicRepositoryTest.cs

@@ -75,6 +75,35 @@ public void LoadTopicGraph_WithAttributes_ReturnsAttributes() {
 
     }
 
+    /*==========================================================================================================================
+    | TEST: LOAD TOPIC GRAPH: WITH NULL ATTRIBUTES: REMOVES ATTRIBUTE
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Calls <see cref="SqlDataReaderExtensions.LoadTopicGraph(IDataReader, Topic?, Boolean?, Boolean)"/> with an <see cref="
+    ///   AttributesDataTable"/> record representing a deleted attribute and confirms that an existing reference topic with that
+    ///   attribute has the value removed.
+    /// </summary>
+    [TestMethod]
+    public void LoadTopicGraph_WithNullAttributes_RemovesAttribute() {
+
+      using var topics          = new TopicsDataTable();
+      using var attributes      = new AttributesDataTable();
+
+      var topic                 = TopicFactory.Create("Root", "Container", 1);
+
+      topic.Attributes.SetValue("Test", "Initial Value");
+
+      topics.AddRow(1, "Root", "Container");
+      attributes.AddRow(1, "Test", null);
+
+      using var tableReader     = new DataTableReader(new DataTable[] { topics, attributes });
+
+      tableReader.LoadTopicGraph(topic);
+
+      Assert.IsNull(topic.Attributes.GetValue("Test"));
+
+    }
+
     /*==========================================================================================================================
     | TEST: LOAD TOPIC GRAPH: WITH RELATIONSHIP: RETURNS RELATIONSHIP
     \-------------------------------------------------------------------------------------------------------------------------*/