fields cannot be reset/emptied in index

[benwalch]

Q	A
Branch	master, 2.x
Bug report?	yes/no
Feature request?	yes/no
BC Break report?	yes/no
RFC?	yes/no

This issue addresses exactly the same case like in #81, but for all types of optional data, where empty means null.
Pimcore returns null for non-required fields. The current implementation leads to data inconsistency between backend and index.

Scenario

Consider having indexed a pimcore data object, with an optional field which has a value.
Now in pimcore backend, the field is emptied / cleared.
With the current implementation, it is impossible that this change will be comitted to the index out of the box (Object getter extractor).
The data in the index won't clear, because of the above issue.