Routers get empty ctrl list on upgrade

[plorenz]

When a cluster was upgraded from 1.2.2 to 1.3.1, the following was observed:

1. All ERs updated to 1.3.1
2. One controller updated to 1.3.1
3. The routers received an empty controller list from the upgraded controller
4. The routers accepted the empty list and were no longer able to communicate with the controllers.

We should do the following:

1. Figure out why the controller sent an empty list and fix it
2. Ignore empty lists when received on the router
3. If the endpoints file is empty on start up, ignore and it fallback to the initial set configured in the router config.