Fix potential NullReferenceException.

ArgumentNotEmptyString should handle null value being passed in.

Author: grokys

src/GitHub.Extensions/Guard.cs

@@ -37,13 +37,13 @@ public static void ArgumentNonNegative(int value, string name)
         }
 
         /// <summary>
-        ///   Checks a string argument to ensure it isn't null or empty.
+        /// Checks a string argument to ensure it isn't null or empty.
         /// </summary>
         /// <param name = "value">The argument value to check.</param>
         /// <param name = "name">The name of the argument.</param>
         public static void ArgumentNotEmptyString(string value, string name)
         {
-            if (value.Length > 0) return;
+            if (value?.Length > 0) return;
             string message = String.Format(CultureInfo.InvariantCulture, "The value for '{0}' must not be empty", name);
 #if DEBUG
             if (!InUnitTestRunner())