Add support for nullable fields

[peel]

Previously all the fields with nullable values were discarded as invalid.
However there is a wide range of fields that cannot be treated as non-nullable.

For example for enrichment failures bad row, we get following structure (trimmed):

                column_name                | is_nullable |          data_type
-------------------------------------------+-------------+-----------------------------
 processor.artifact                        | NO          | character varying
 processor.version                         | NO          | character varying
 failure.messages                          | YES         | jsonb
 failure.timestamp                         | YES         | timestamp without time zone
 payload.enriched.app_id                   | YES         | character varying
 payload.enriched.base_currency            | YES         | character varying
...

Where only processor is non-nullable. Therefore enrichment failures would not be written with evaluations like:

payload.enriched.br_cookies -> Some(null) ->  BigInt -> Left(Invalid type BIGINT for value Some(null))