Implemented a nil decoder for kafka (#45)

kafka/encoder.go

@@ -1,5 +1,8 @@
 package kafka
 
+// NilDecoder is a decoder that always returns a nil, no matter the input.
+var NilDecoder DecoderFunc = func([]byte) (interface{}, error) { return nil, nil }
+
 // Decoder represents a Kafka data decoder.
 type Decoder interface {
 	// Decode transforms byte data to the desired type.

kafka/encoder_test.go

@@ -113,3 +113,23 @@ func TestEncoderFunc_Encode(t *testing.T) {
 	assert.Equal(t, b, result)
 	assert.True(t, e == err, "Received error is not exactly the same object that was returned by the function.")
 }
+
+func TestNilDecoder(t *testing.T) {
+	tests := []struct {
+		name string
+		in   []byte
+	}{
+		{name: "some bytes", in: []byte("test")},
+		{name: "empty bytes", in: []byte{}},
+		{name: "nil", in: nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := kafka.NilDecoder.Decode(tt.in)
+
+			assert.NoError(t, err)
+			assert.Nil(t, got)
+		})
+	}
+}