feat: Empty json output to array

[stepanstipl]

This PR makes an empty output (i.e. no issues found) in json format to be an empty array (instead of null). This is more friendly for CI use (#31), such as with jq.

Before:

$./bin/kubent-darwin-amd64 -o json                                                                                                                       2
2:36PM INF >>> Kube No Trouble `kubent` <<<
...
...
null

After:

$./bin/kubent-darwin-amd64 -o json                                                                                                                     130
3:09PM INF >>> Kube No Trouble `kubent` <<<
...
...
[]

[ghost]

LGTM