Improve resiliency against malformed json in monitoring.json

[jhg03a]

When upgrading to a release that contains issue #3104, there is a brief window in which TO will return an invalid json object of {} until the first snapshot. If TM polls during this window it will flag all caches as offline. This potentially can manifest all the way up to TR which will cause 503 errors on client steering delivery services that nolonger have valid targets similarly to #3521.

TM should perform additional sanity checks on monitoring.json to ensure root json entries are present and subarrays are not empty.