fix: consistent handling of nil passed to any wrapping function

errors.go

@@ -44,6 +44,10 @@ func New(message string, details ...any) error {
 
 // Wrap returns a new errkitError that has the given message and err as the cause.
 func Wrap(err error, message string, details ...any) error {
+	if err == nil {
+		return nil
+	}
+
 	e := newError(errors.New(message), 2, details...)
 	e.cause = err
 	return e

errors_test.go

@@ -286,6 +286,16 @@ func TestErrorsWrapping(t *testing.T) {
 		if wrappedErr != nil {
 			t.Errorf("nil expected to be returned")
 		}
+
+		wrappedErr = errkit.Wrap(nil, "Some message")
+		if wrappedErr != nil {
+			t.Errorf("nil expected to be returned")
+		}
+
+		wrappedErr = errkit.WithStack(nil)
+		if wrappedErr != nil {
+			t.Errorf("nil expected to be returned")
+		}
 	})
 }