More explicit error on empty JSON bodies (#459)

Return a more understandable error when the JSON codec attempts to
unmarshal an empty body.

Fixes #452.

codec.go

@@ -15,6 +15,7 @@
 package connect
 
 import (
+	"errors"
 	"fmt"
 
 	"google.golang.org/protobuf/encoding/protojson"
@@ -93,6 +94,9 @@ func (c *protoJSONCodec) Unmarshal(binary []byte, message any) error {
 	if !ok {
 		return errNotProto(message)
 	}
+	if len(binary) == 0 {
+		return errors.New("zero-length payload is not a valid JSON object")
+	}
 	var options protojson.UnmarshalOptions
 	return options.Unmarshal(binary, protoMessage)
 }

codec_test.go

@@ -0,0 +1,37 @@
+// Copyright 2021-2023 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package connect
+
+import (
+	"strings"
+	"testing"
+
+	"connectrpc.com/connect/internal/assert"
+	"google.golang.org/protobuf/types/known/emptypb"
+)
+
+func TestJSONCodec(t *testing.T) {
+	t.Parallel()
+
+	var empty emptypb.Empty
+	codec := &protoJSONCodec{name: "json"}
+	err := codec.Unmarshal([]byte{}, &empty)
+	assert.NotNil(t, err)
+	assert.True(
+		t,
+		strings.Contains(err.Error(), "valid JSON"),
+		assert.Sprintf(`error message should explain that "" is not a valid JSON object`),
+	)
+}