allow choice recognizer to accept null and empty string

libraries/Microsoft.Bot.Builder.Dialogs/Choices/ChoiceRecognizers.cs

@@ -17,6 +17,11 @@ public static List<ModelResult<FoundChoice>> RecognizeChoices(string utterance,
 
         public static List<ModelResult<FoundChoice>> RecognizeChoices(string utterance, IList<Choice> list, FindChoicesOptions options = null)
         {
+            if (utterance == null)
+            {
+                utterance = string.Empty;
+            }
+
             // Try finding choices by text search first
             // - We only want to use a single strategy for returning results to avoid issues where utterances
             //   like the "the third one" or "the red one" or "the first division book" would miss-recognize as

libraries/Microsoft.Bot.Builder.Dialogs/Choices/Find.cs

@@ -21,11 +21,6 @@ public static List<ModelResult<FoundChoice>> FindChoices(string utterance, IList
 
         public static List<ModelResult<FoundChoice>> FindChoices(string utterance, IList<Choice> choices, FindChoicesOptions options = null)
         {
-            if (string.IsNullOrEmpty(utterance))
-            {
-                throw new ArgumentNullException(nameof(utterance));
-            }
-
             if (choices == null)
             {
                 throw new ArgumentNullException(nameof(choices));

tests/Microsoft.Bot.Builder.Dialogs.Tests/ChoicesRecognizersTests.cs

@@ -168,6 +168,13 @@ public void ShouldFindMultipleChoicesInAnUtteranceByNumerical_index()
             AssertChoice(found[1], "blue", 2, 1.0f);
         }
 
+        [TestMethod]
+        public void ShouldAcceptNullForUtterance()
+        {
+            var found = ChoiceRecognizers.RecognizeChoices(null, colorChoices);
+            Assert.AreEqual(0, found.Count);
+        }
+
         // Helper functions
 
         private static void AssertResult<T>(ModelResult<T> result, int start, int end, string text)