[bugfix] Fix EmptyJSONObject/EmptyJSONArray (#2576)

* Fix EmptyJSONObject/EmptyJSONArray

These are meant to be the bytes representing an empty object and array in JSON: `{}` and `[]`. They are actually the strings `"{}"` and `"[]"`. This causes clients expecting an object or array to not be able to parse the response.

* Use json.RawMessage instead of []byte

internal/api/util/response.go

@@ -51,8 +51,8 @@ var (
 	ErrorRateLimited = mustJSON(map[string]string{
 		"error": "rate limit reached",
 	})
-	EmptyJSONObject = mustJSON("{}")
-	EmptyJSONArray  = mustJSON("[]")
+	EmptyJSONObject = json.RawMessage(`{}`)
+	EmptyJSONArray  = json.RawMessage(`[]`)
 
 	// write buffer pool.
 	bufPool sync.Pool