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

[Copilot]

Description

The Uri(Uri, Uri) constructor threw NullReferenceException when passed a null relativeUri, instead of ArgumentNullException at parameter validation time.

Changes:

• Added ArgumentNullException.ThrowIfNull(relativeUri) in constructor (Uri.cs:598)
• Added test case Uri_Ctor_NullRelativeUri_ThrowsArgumentNullException in UriParameterValidationTest.cs using AssertExtensions.Throws to verify both exception type and parameter name
• Added using System.Tests; import for AssertExtensions test utilities

// Before: throws NullReferenceException deep in CreateThisFromUri
// After: throws ArgumentNullException immediately
Uri baseUri = new Uri("http://localhost/");
Uri result = new Uri(baseUri, (Uri)null);  // ArgumentNullException with correct parameter name

Customer Impact

Applications relying on proper exception types for error handling would catch NullReferenceException instead of expected ArgumentNullException, making debugging harder and potentially masking programming errors.

Regression

No. This is a latent bug in the API.

Testing

Unit test added to verify ArgumentNullException is thrown with correct parameter name. Test uses AssertExtensions.Throws for stronger validation, matching patterns used in other test files (e.g., UriBuilderTests.cs, UriEscapingTest.cs). Existing Uri tests continue to pass.

Risk

Minimal. Single-line parameter validation added at entry point, following identical pattern used for baseUri parameter validation on the preceding line. No behavioral changes to valid inputs.

Original prompt

The Uri constructor that accepts two Uri arguments doesn't properly validate that the second argument is not null, leading to a NRE instead of an ArgumentNullException being thrown.
Add the null validation and add a test for this case.

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[Copilot]

Pull request overview

This pull request fixes a bug where the Uri(Uri, Uri) constructor threw NullReferenceException instead of ArgumentNullException when passed a null relativeUri parameter.

Changes:

• Added null validation for the relativeUri parameter in the Uri(Uri, Uri) constructor
• Added a test case to verify that ArgumentNullException is thrown for null relativeUri

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.Uri/src/System/Uri.cs	Added ArgumentNullException.ThrowIfNull(relativeUri) validation in the Uri(Uri, Uri) constructor immediately after the existing baseUri validation
src/libraries/System.Private.Uri/tests/FunctionalTests/UriParameterValidationTest.cs	Added test case Uri_Ctor_NullRelativeUri_ThrowsArgumentNullException to verify proper exception type is thrown