Add support for null_value on GeoPoint property

[russcam]

Relates: elastic/elasticsearch#29451
Documentation: https://www.elastic.co/guide/en/elasticsearch/reference/6.3/geo-point.html#geo-point-params

Add support for null_value property on GeoPoint field type mapping

[russcam]

Closing this as #3364 has been merged into 6.3