v1.7.0

Write a sentinel for a null, so a file can round-trip its nulls.

New

• CsvConfig(nullPlaceholder: 'NULL') writes that text for a null instead of
  an empty field. Some destinations want a sentinel: Postgres COPY reads
  \N, and plenty of exports use NULL. Paired with nullValues the same
  file reads back with its nulls intact, which the previous release could
  decode but not produce.
• The placeholder is written with necessary quoting whatever quoteMode is
  set to. It is a sentinel rather than data, and under always or strings it
  would come out quoted, which nullValues deliberately never matches, so the
  round trip would have quietly failed. It still gets quotes when the text
  itself needs them.
• An empty string is untouched by all of this. It still encodes as a quoted
  empty field, so '' and null stay distinguishable on the way back.