Fix up null check in Timestamps.TryParse

Signed-off-by: Safia Abdalla <safia@safia.rocks>

src/CloudNative.CloudEvents/Timestamps.cs

@@ -44,13 +44,16 @@ namespace CloudNative.CloudEvents
         /// <summary>
         /// Attempts to parse a string as an RFC-3339-formatted date/time and UTC offset.
         /// </summary>
-        /// <param name="input"></param>
-        /// <param name="result"></param>
-        /// <returns></returns>
+        /// <param name="input">A string to be parsed as a <see cref="DateTimeOffset"/>.</param>
+        /// <param name="result">A <see cref="DateTimeOffset"/> parsed from the <paramref name="input"/>.</param>
+        /// <returns><see langword="true"/> if <paramref name="input"/> was parsed successfully, <see langword="false"/> otherwise.</returns>
         public static bool TryParse(string input, out DateTimeOffset result)
         {
-            // TODO: Check this and add a test
-            Validation.CheckNotNull(input, nameof(input));
+            if (string.IsNullOrEmpty(input))
+            {
+                result = default;
+                return false;
+            }
 
             if (input.Length < MinLength) // "yyyy-MM-ddTHH:mm:ssZ" is the shortest possible value.
             {

test/CloudNative.CloudEvents.UnitTests/TimestampsTest.cs

@@ -1,4 +1,4 @@
-// Copyright 2021 Cloud Native Foundation. 
+// Copyright 2021 Cloud Native Foundation.
 // Licensed under the Apache 2.0 license.
 // See LICENSE file in the project root for full license information.
 
@@ -89,6 +89,7 @@ namespace CloudNative.CloudEvents.UnitTests
         }
 
         [Theory]
+        [InlineData(null)]
         [InlineData("")]
         [InlineData("garbage")]
         [InlineData("garbage that is long enough")]