Allow null arguments in ProcessStartInfo constructor and Process.Start (dotnet#126076)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Author: Copilot

src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs

@@ -160,7 +160,7 @@ public void Refresh() { }
         [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("ios")]
         [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("tvos")]
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("maccatalyst")] // this needs to come after the ios attribute due to limitations in the platform analyzer
-        public static System.Diagnostics.Process Start(string fileName, string arguments) { throw null; }
+        public static System.Diagnostics.Process Start(string fileName, string? arguments) { throw null; }
         [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("ios")]
         [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("tvos")]
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("maccatalyst")] // this needs to come after the ios attribute due to limitations in the platform analyzer
@@ -221,7 +221,7 @@ public sealed partial class ProcessStartInfo
     {
         public ProcessStartInfo() { }
         public ProcessStartInfo(string fileName) { }
-        public ProcessStartInfo(string fileName, string arguments) { }
+        public ProcessStartInfo(string fileName, string? arguments) { }
         public ProcessStartInfo(string fileName, System.Collections.Generic.IEnumerable<string> arguments) { }
         public System.Collections.ObjectModel.Collection<string> ArgumentList { get { throw null; } }
         [System.Diagnostics.CodeAnalysis.AllowNullAttribute]

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs

@@ -1313,7 +1313,7 @@ public static Process Start(string fileName)
         [UnsupportedOSPlatform("ios")]
         [UnsupportedOSPlatform("tvos")]
         [SupportedOSPlatform("maccatalyst")]
-        public static Process Start(string fileName, string arguments)
+        public static Process Start(string fileName, string? arguments)
         {
             // the underlying Start method can only return null on Windows platforms,
             // when the ProcessStartInfo.UseShellExecute property is set to true.

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.cs

@@ -49,7 +49,7 @@ public ProcessStartInfo(string fileName)
         ///     Specifies the name of the application that is to be started, as well as a set
         ///     of command line arguments to pass to the application.
         /// </devdoc>
-        public ProcessStartInfo(string fileName, string arguments)
+        public ProcessStartInfo(string fileName, string? arguments)
         {
             _fileName = fileName;
             _arguments = arguments;

src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs

@@ -448,6 +448,17 @@ public void TestArgumentsProperty()
             Assert.Equal("-arg3 -arg4", psi.Arguments);
         }
 
+        [Fact]
+        public void TestArgumentsNullProperty()
+        {
+            string? args = null;
+            ProcessStartInfo psi = new ProcessStartInfo("filename", args);
+            Assert.Equal(string.Empty, psi.Arguments);
+
+            psi.Arguments = null;
+            Assert.Equal(string.Empty, psi.Arguments);
+        }
+
         [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported)), InlineData(true), InlineData(false)]
         public void TestCreateNoWindowProperty(bool value)
         {