Update ResultTests to check for non-null empty Errors

Modified assertions in ResultTests.cs to ensure the Errors
property is not null but an empty collection when the result
is successful. This aligns with the updated Result class
implementation where Errors is initialized as an empty
collection instead of being null.

Author: iPazooki

DomainValidation.UnitTests/ResultTests.cs

@@ -39,23 +39,26 @@ public void Result_ConstructorWithMessage_Success_ReturnsIsSuccessTrue()
     {
         var result = new Result(true, "Operation successful");
         Assert.True(result.IsSuccess);
-        Assert.Null(result.Errors);
+        Assert.NotNull(result.Errors);
+        Assert.Empty(result.Errors);
     }
 
     [Fact]
     public void Result_ConstructorErrorNone_Success_ReturnsIsSuccessTrue()
     {
         var result = new Result(true, Error.None);
         Assert.True(result.IsSuccess);
-        Assert.Null(result.Errors);
+        Assert.NotNull(result.Errors);
+        Assert.Empty(result.Errors);
     }
 
     [Fact]
     public void Result_ConstructorWithoutMessage_Success_ReturnsIsSuccessTrue()
     {
         var result = new Result(true);
         Assert.True(result.IsSuccess);
-        Assert.Null(result.Errors);
+        Assert.NotNull(result.Errors);
+        Assert.Empty(result.Errors);
     }
 
     [Fact]