refactor(results): update Result class for nullable types

- Adjusts the Result class to enforce non-nullable constraints on generic types.
- Simplifies the results tests by directly checking for null instead of multiple assertions.
- Updates method signatures for better type safety and clarity.

Author: winromulus

src/ES.FX/Results/Result.cs

@@ -34,7 +34,7 @@ public Result(Problem problem) : base(problem)
 /// </summary>
 /// <typeparam name="T">Type of result</typeparam>
 [PublicAPI]
-public class Result<T> : IResult
+public class Result<T> : IResult where T : notnull
 {
     private readonly Problem? _problem;
     private readonly T? _result;

tests/ES.FX.Tests/Results/ResultsTests.cs

@@ -10,17 +10,7 @@ public class ResultsTests
     public void TypedResult_CanBe_Null()
     {
         var result = TypedResultAsNull();
-        Assert.True(result.IsResult);
-
-        if(result.TryPickResult(out var resultValue))
-        {
-            Assert.Null(resultValue);
-        }
-
-        if (!result.TryPickProblem(out var problemValue))
-        {
-            Assert.Null(problemValue);
-        }
+        Assert.Null(result);
     }
 
     [Fact]
@@ -121,7 +111,7 @@ public void Results_CanBeEqualityChecked()
 
     private static Result<string> TypedResultAsProblem() => new Problem();
     private static Result<string> TypedResultAsValue() => string.Empty;
-    private static Result<object?> TypedResultAsNull() => (object?)null;
+    private static Result<object>? TypedResultAsNull() => null;
 
 
 }