devDependencies sometimes omitted from object

[keis]

devDependencies is not set on the object unless it's in package.json unlike dependencies and peerDependencies which is always present.

running read-installed with a stub package.json like {"name": "foobar"} shows this behavior.

It would be great if this was instead set to a empty object like the others for consistency and also ease of iterating over the dev-dependencies.