GEOMESA-328 Fix IndexIterator exception

     * The fix is from @anthony-ccri.
     * The tests show the exceptions which we observed on tablet servers.
This is not strictly needed to address this issue, but it DID partially get us into trouble. And I've fixed it to match the behavior of getDtgDescriptor -- if the hint points to an attribute that doesn't exist, `None` is returned.

If I had done this originally, we would not get NPEs from wrappedSTFilter's setAttribute call. However, the filter would still remove all events if the ECQL included a temporal predicate. So the rest of this PR is still needed.