JSON Unmarshal fails if refID is an empty string.

[asimpson]

What happened:
Queries that do not have a refID but instead have an empty string fail to unmarshal. This is most likely because jsoniter uses an empty string to denote the end of parsing which is problematic considering "" is a valid property name/key in JSON.

What you expected to happen:
Unmarshaling should work even if the refID is empty, encoding/json handles this case: https://play.golang.com/p/KhNyYGxA7eV

How to reproduce it (as minimally and precisely as possible):
I made and pushed a failing unit test here:

grafana-plugin-sdk-go/backend/json_test.go

Lines 147 to 157 in fee7ea4

	func TestQueryDataWithoutRefID(t *testing.T) {
	qdr := backend.NewQueryDataResponse()
	qdr.Responses[""] = testDataResponse()
	b, err := json.Marshal(qdr)
	require.NoError(t, err)
	respCopy := &backend.QueryDataResponse{}
	//Now unmarshall without a refId
	err = json.Unmarshal(b, respCopy)
	require.NoError(t, err)
	}