GEOMESA-330 Avro SimpleFeature's getProperty should return a Property even when the value is null

This addresses an issue found by a user who was attempting to run the Quick Start tutorial.
GEOMESA-328 Fix IndexIterator exception
 * The fix is from @anthony-ccri.
 * The tests show the exceptions which we observed on tablet servers.
Merge branch 'accumulo1.5.x/1.x' of github.com:locationtech/geomesa into f_knn_search