.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/python-api.rst

@@ -1279,6 +1279,8 @@ To create a species record with a note on when it was first seen, you can use th
 
 The first time this is called the record will be created for ``name="Palm"``. Any subsequent calls with that name will ignore the second argument, even if it includes different values.
 
+``None`` values are matched correctly: calling ``.lookup()`` a second time with the same values will return the primary key of the existing row even if some of those values are ``None``.
+
 ``.lookup()`` also accepts keyword arguments, which are passed through to the :ref:`insert() method <python_api_creating_tables>` and can be used to influence the shape of the created table. Supported parameters are:
 
 - ``pk`` - which defaults to ``id``
@@ -1324,6 +1326,8 @@ To extract the ``species`` column out to a separate ``Species`` table, you can d
         "species": "Common Juniper"
     }, extracts={"species": "Species"})
 
+``None`` values are not extracted: no record is created for them in the lookup table and the column value stays ``null``.
+
 .. _python_api_m2m:
 
 Working with many-to-many relationships
@@ -2022,6 +2026,8 @@ This produces a lookup table like so:
         "latin" TEXT
     )
 
+Rows where every extracted column is ``null`` are not extracted: no record is created for them in the lookup table and their foreign key column is left as ``null``. When extracting multiple columns, rows where at least one of the extracted columns has a value will be extracted as usual.
+
 .. _python_api_hash:
 
 Setting an ID based on the hash of the row contents