PHP-1351: Handle listCollections and listIndexes results for non-existent databases and collections

https://jira.mongodb.org/browse/PHP-1351
For a non-existent database, `listCollections` returns a cursor response with no cursor ID and an empty first batch. Previously, this resulted in the driver returning `NULL` from this conditional. I think the `for` loop below is easy to read and will ensure an empty array is returned in that case.