Fix standard compatiblility

Non-nil but empty map with omitempty should be ignored.

Signed-off-by: Peng Gao <peng.gao.dut@gmail.com>

compatible_test.go

@@ -18,3 +18,23 @@ func TestEncoderHasTrailingNewline(t *testing.T) {
 	stdenc.Encode(1)
 	should.Equal(stdbuf.Bytes(), buf.Bytes())
 }
+
+// Non-nil but empty map should be ignored.
+func TestOmitempty(t *testing.T) {
+	o := struct {
+		A           string            `json:"a,omitempty"`
+		B           string            `json:"b,omitempty"`
+		Annotations map[string]string `json:"annotations,omitempty"`
+	}{
+		A:           "a",
+		B:           "b",
+		Annotations: map[string]string{},
+	}
+	should := require.New(t)
+	var buf, stdbuf bytes.Buffer
+	enc := ConfigCompatibleWithStandardLibrary.NewEncoder(&buf)
+	enc.Encode(o)
+	stdenc := json.NewEncoder(&stdbuf)
+	stdenc.Encode(o)
+	should.Equal(string(stdbuf.Bytes()), string(buf.Bytes()))
+}