prometheus stats endpoints 500s without machines or pods

[Alberto Donato]

If no machines and/or no PODs are registered in MAAS, the prometheus stats endpoint returns a 500 error.

$ curl -v http://localhost:5240/MAAS/stats
* Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 5240 (#0)
> GET /MAAS/stats HTTP/1.1
> Host: localhost:5240
> User-Agent: curl/7.58.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Server: TwistedWeb/17.9.0
< Date: Fri, 25 Jan 2019 10:59:00 GMT
< Content-Type: text/plain; charset=utf-8
< X-Frame-Options: SAMEORIGIN
< Content-Length: 61
<
* Connection #0 to host localhost left intact
float() argument must be a string or a number, not 'NoneType'