.extract() and .lookup() no longer extract null values, closes #186

- table.extract() and the sqlite-utils extract command now skip rows
  where every extracted column is null: the new foreign key column is
  left null and no all-null record is added to the lookup table. Rows
  with at least one non-null extracted column are extracted as before.
- The extracts= option to insert() and friends keeps None values as
  null instead of creating a lookup record for them - previously each
  insert batch added a duplicate null row to the lookup table.
- table.lookup() compares values using IS rather than = so lookup
  values containing None match existing rows correctly, instead of
  inserting a duplicate row on every call.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/changelog.rst

@@ -9,6 +9,9 @@
 Unreleased
 ----------
 
+- **Breaking change**: ``table.extract()`` - and the ``sqlite-utils extract`` command - no longer extract rows where every extracted column is ``null``. Those rows now keep a ``null`` value in the new foreign key column instead of pointing at an all-``null`` record in the lookup table. When extracting multiple columns, rows are still extracted if at least one of the columns has a value. (:issue:`186`)
+- The ``extracts=`` option to ``table.insert()`` and friends no longer creates a lookup table record for ``None`` values - the column value stays ``null``. Previously every batch of inserted rows containing a ``None`` value would add a duplicate ``null`` record to the lookup table.
+- Fixed a bug where ``table.lookup()`` inserted a duplicate row on every call if any of the lookup values were ``None``. Lookup values are now compared using ``IS`` so that ``None`` values match existing rows correctly.
 - JSON output from the command-line tool no longer escapes non-ASCII characters, so ``sqlite-utils data.db "select '日本語' as text"`` now outputs ``[{"text": "日本語"}]``. This matches how values were already stored by ``insert`` and how CSV/TSV output already behaved. A new ``--ascii`` option restores the previous behavior of escaping non-ASCII characters, for output destinations that cannot handle UTF-8 - see :ref:`cli_query_json_ascii`. The option is available on the ``query``, ``rows``, ``search``, ``tables``, ``views``, ``triggers``, ``indexes`` and ``memory`` commands. The ``convert --multi --dry-run`` preview and ``plugins`` output also no longer escape non-ASCII characters. (:issue:`625`)
 - ``table.insert_all(..., pk=...)`` now raises ``InvalidColumns`` if ``pk=`` names columns that do not exist in an existing table. Previously this behaved inconsistently, with single-row inserts raising a ``KeyError`` while other row counts succeeded. (:issue:`732`)
 - Fixed an ``IndexError`` from ``table.insert(..., pk=..., ignore=True)`` when an ignored insert followed writes to another table on the same connection. ``last_pk`` is now populated from the explicit primary key value instead of looking up a stale ``lastrowid``. (:issue:`554`)