PHP-1355 and PHP-1357: Fix collection and index enumeration for empty first batch

 * https://jira.mongodb.org/browse/PHP-1355
 * https://jira.mongodb.org/browse/PHP-1357
@derickr: What do you think about making the cursor struct's query field a null zval when we initialize it from the command document (instead of a null pointer)? That would obviate the need for patching `log_stream.c` here, and also remove some logic from the `info()` method in `cursor_shared.c`. I expect there are a few places in the code where we check if `cursor->query` is null, which would need to be changed.