Feature request: turn empty string to postgres null in exports

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At the moment, _json_to_pqtext as a format will output an empty string on an empty field. For some workloads, it'd make sense to translate JSON NULL and empty strings to a postgres NULL.

This function could be called _json_to_pqtext_null for example.

In general, the question is how to add options to handling the different possibilities of JSON NULL, empty field and missing field to schaufel.