Do not print empty Attr

This is described in Handler.Handle method: If an Attr's key and value are both the zero value, ignore the Attr.

encoding.go

@@ -97,6 +97,10 @@ func (e encoder) writeMessage(buf *buffer, level slog.Level, msg string) {
 }
 
 func (e encoder) writeAttr(buf *buffer, a slog.Attr, group string) {
+	// Elide empty Attrs.
+	if a.Equal(slog.Attr{}) {
+		return
+	}
 	value := a.Value.Resolve()
 	if value.Kind() == slog.KindGroup {
 		subgroup := a.Key

handler_test.go

@@ -62,6 +62,8 @@ func TestHandler_Attr(t *testing.T) {
 		slog.Any("err", errors.New("the error")),
 		slog.Any("stringer", theStringer{}),
 		slog.Any("nostringer", noStringer{Foo: "bar"}),
+		slog.Attr{},
+		slog.Any("", nil),
 	)
 	AssertNoError(t, h.Handle(context.Background(), rec))