#2708 add Add a judgment to the graph constructor (#2709)

* #2708 add Add a judgment to the graph constructor

* #2708 add Add a judgment to the graph constructor & added unit test

* #2708 #2079 move GraphTests to AutoGen.Tests; delete AutoGen.Core.Tests project

dotnet/AutoGen.sln

@@ -1,4 +1,4 @@
-
+
 Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 17
 VisualStudioVersion = 17.8.34322.80
@@ -206,4 +206,4 @@ Global
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {93384647-528D-46C8-922C-8DB36A382F0B}
 	EndGlobalSection
-EndGlobal
+EndGlobal

dotnet/src/AutoGen.Core/GroupChat/Graph.cs

@@ -12,9 +12,16 @@ public class Graph
 {
     private readonly List<Transition> transitions = new List<Transition>();
 
-    public Graph(IEnumerable<Transition> transitions)
+    public Graph()
     {
-        this.transitions.AddRange(transitions);
+    }
+
+    public Graph(IEnumerable<Transition>? transitions)
+    {
+        if (transitions != null)
+        {
+            this.transitions.AddRange(transitions);
+        }
     }
 
     public void AddTransition(Transition transition)

dotnet/test/AutoGen.Tests/GroupChat/GraphTests.cs

@@ -0,0 +1,18 @@
+
+using Xunit;
+
+namespace AutoGen.Tests
+{
+    public class GraphTests
+    {
+        [Fact]
+        public void GraphTest()
+        {
+            var graph1 = new Graph();
+            Assert.NotNull(graph1);
+
+            var graph2 = new Graph(null);
+            Assert.NotNull(graph2);
+        }
+    }
+}