Make splainer robust against debug=null returned by a search engine proxy

[mkr]

splainer (or rather: Quepid) fails when running it against a search engine proxy which adds a debug: null to its response. It would be nice, if splainer would handle this gracefully

{
  "responseHeader": {
    "status": 0,
    ...
    }
  },
  "response": {
    "numFound": 17535,
    "start": 0,
    "docs": [
    ...
    ]
  },
  "debug": null
}