Take care of non-existing repos
Take care of non-existing repos

`get_repo` returns {"name":"get_repo", "result": "null"} for a non-existing
repo request, and this make JSON.parse() a little angry trying to parse "null"
as-is:

`ERROR: nexus3_repo[test] (nexus3-criteo::repositories line 2) had an error:
JSON::ParserError: 757: unexpected token at 'null'`

criteo-cookbooks/nexus3,criteo-cookbooks/nexus3