check for collections field in db.stats output

[dipu018]

fix for Splunk support case 3015833

[dloucasfx]

Thanks for your PR.
The plugin does not support sharded cluster, but with your change, we'll be able to collect non-sharded related metrics

For reference, this is how the db.stats() looks like on a sharded cluster

{
  raw: {
    'shard0/shard0-replica0:27017,shard0-replica1:27017,shard0-replica2:27017': {
      db: 'test',
      collections: 0,
      views: 0,
      objects: 0,
      avgObjSize: 0,
      dataSize: 0,
      storageSize: 0,
      totalSize: 0,
      indexes: 0,
      indexSize: 0,
      scaleFactor: 1,
      fileSize: 0,
      fsUsedSize: 0,
      fsTotalSize: 0,
      ok: 1
    }
  },
  objects: 0,
  avgObjSize: 0,
  dataSize: 0,
  storageSize: 0,
  totalSize: 0,
  indexes: 0,
  indexSize: 0,
  scaleFactor: 1,
  fileSize: 0,
  ok: 1,
  '$clusterTime': {
    clusterTime: Timestamp({ t: 1658417413, i: 4 }),
    signature: {
      hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
      keyId: Long("0")
    }
  },
  operationTime: Timestamp({ t: 1658417412, i: 3 })
}