RestDataSource: If messages object has `__default__` set to `""` an error is generated

[andrewrmiller]

If you have a messages object in your dataSource.json that looks like this:

messages: {
   myMessageObject: {
    val1: "Foo",
    val2: "Bar",
    __default__: ""
  }
}

An error will be generated the the message patching code because "" is falsy.