set key to default "" in order for iterator not to fail

[Jojoooo1]

async getHistoryForKey (key) {
   return await this.handler.handleGetHistoryForKey(key, this.channel_id, this.txId);
}

if key does not exist it will return a MockHistoryQueryIterator where data is undefined { data: undefined, [...] } which will cause errors when executing the iterator.

[sneljo1]

Is this the behaviour of the actual chaincode? or should this be an empty array?