GEOMESA-328 Fix IndexIterator exception

     * The fix is from @anthony-ccri.
     * The tests show the exceptions which we observed on tablet servers.
Perhaps more readable?

```scala
def getDtgFieldName(sft: SimpleFeatureType) = 
   for {
      attr <- Option(sft.getUserData.get(SF_PROPERTY_NAME)
      attrName <- attr.getName if sft.getDescriptor(attrName)
   } yield attrName
```