Add null validation for relativeUri in Uri(Uri, Uri) constructor (#123134)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: MihaZupan <25307628+MihaZupan@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.Uri/src/System/Uri.cs

@@ -595,6 +595,7 @@ private void CreateUri(Uri baseUri, string? relativeUri, bool dontEscape)
         public Uri(Uri baseUri, Uri relativeUri)
         {
             ArgumentNullException.ThrowIfNull(baseUri);
+            ArgumentNullException.ThrowIfNull(relativeUri);
 
             if (!baseUri.IsAbsoluteUri)
                 throw new ArgumentOutOfRangeException(nameof(baseUri));

src/libraries/System.Private.Uri/tests/FunctionalTests/UriParameterValidationTest.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Tests;
 using Xunit;
 
 namespace System.PrivateUri.Tests
@@ -32,5 +33,12 @@ public void Uri_IsBaseOf_NullParameter_ThrowsArgumentException()
             Uri baseUri = new Uri("http://localhost/");
             Assert.Throws<ArgumentNullException>(() => baseUri.IsBaseOf((Uri)null));
         }
+
+        [Fact]
+        public void Uri_Ctor_NullRelativeUri_ThrowsArgumentNullException()
+        {
+            Uri baseUri = new Uri("http://localhost/");
+            AssertExtensions.Throws<ArgumentNullException>("relativeUri", () => new Uri(baseUri, (Uri)null));
+        }
     }
 }