set selectedDevice to null instead of undefined

Summary:
We are using `redux-persist` to persist parts of our redux store over reloads. In this library, there is a check if a value "was removed to the store". However, they are doing this check by checking if the value of this key is `undefined`. This is not a good way of checking if a property exists on an object, because it can exists but explicitly be set to `undefined`. This was the case for `connections.selectedDevice`, we were setting the value to undefined, once the device disconnected. This caused the key to be persisted, eventhough it wasn't whitelisted for persisting.

In this diff, we are setting the `selectedDevice` to `null` instead of `undefined` once a device disconnects.

Reviewed By: passy

Differential Revision: D16121260

fbshipit-source-id: f32c8ea9e30f02e065fa63380ae0245379a47496

src/reducers/connections.js

@@ -189,7 +189,7 @@ const reducer = (state: State = INITAL_STATE, action: Action): State => {
       let selection = {};
       if (selectedDeviceWasRemoved) {
         selection = {
-          selectedDevice: devices[devices.length - 1],
+          selectedDevice: devices[devices.length - 1] || null,
           selectedApp: null,
           selectedPlugin: DEFAULT_PLUGIN,
         };