make eventdto.enddate nullable

BeckmanComms.Api.DataTransferObjects/BeckmanComms.Api.DataTransferObjects.nuspec

@@ -13,6 +13,7 @@
     <releaseNotes>
       unreleased:
       * Add nullable EventDto recurrence ID.
+      * Make EventDto.EndDate nullable.
       
       2.0.0-alpha08:
       * Add group type property to GroupDto.

BeckmanComms.Api.DataTransferObjects/V2/Events/EventDto.cs

@@ -35,7 +35,7 @@ namespace BeckmanComms.Api.DataTransferObjects.V2.Events
         /// <summary>
         /// Ending date and time of event.
         /// </summary>
-        public DateTime EndDate { get; set; }
+        public DateTime? EndDate { get; set; }
 
         /// <summary>
         /// Event location.