Add nullable context to SabreTools.DatItems

This change also starts migrating the internals of the DatItem formats to the new internal models. Right now, it's basically just acting like a wrapper around those models.

SabreTools.Core/ArrayExtensions.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace SabreTools.Core
+{
+    public static class ArrayExtensions
+    {
+        /// <summary>
+        /// Indicates whether the specified array is null or has a length of zero
+        /// </summary>
+        public static bool IsNullOrEmpty(this Array? array)
+        {
+            return array == null || array.Length == 0;
+        }
+    }
+}