Import sets properties to "null" if a row is missing data

[sleidig]

Describe the bug
The import UI works well if all rows have data for all mapped columns, if one row is missing details (e.g. the "gender" column being empty for that person), the document in the Aam database ends up with that property explicitly set to "null". That screws up some of our filters and displays like dashboard widgets.

Example db doc:

  "dateOfBirth": "2007-10-23",
  "name": "John Doe,
  "gender": null,

The expected doc after import should probably simply not include the "gender" property.

[sleidig]

with the re-implementation #1916 this problem has been implicitly resolved