CSV Import - Option for NULL value representation

The importer expects `NULL` values to be represented by an "empty value" (I did not test what happens with a quoted empty string).

Common representation for these values are also `NULL`, `\N` (postgres default).

Postico should have an option for specifying the `NULL` value representation.

Start by locating Postico's CSV import entry point and the existing handling for empty values. Confirm how NULL, an empty value, and PostgreSQL's \N representation are currently interpreted; done means the importer offers a selectable NULL representation and preserves the chosen behavior during import.

Written by the indexing model from the issue text.