in extract code, check equality with IS instead of = for nulls

[fgregg]

sqlite "IS" is equivalent to SQL "IS NOT DISTINCT FROM"

closes #423

[codecov]

Codecov Report

Merging #455 (1b35a92) into main (855bce8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #455   +/-   ##
=======================================
  Coverage   96.64%   96.64%           
=======================================
  Files           6        6           
  Lines        2566     2566           
=======================================
  Hits         2480     2480           
  Misses         86       86           

Impacted Files	Coverage Δ
sqlite_utils/db.py	97.54% <ø> (ø)

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[simonw]

Can you add a new test for this? Something derived from the example in #423 would work great.

[simonw]

I did not know is worked against strings and integers and booleans!

https://latest.datasette.io/_memory?sql=select+1+is+1%2C+2.3+is+2.3%2C+true+is+true%2C+true+is+false%2C+%27dog%27+is+%27dog%27%2C+%27dog%27+is+%27cat%27%2C+null+is+null%2C+null+is+not+null

select 1 is 1, 2.3 is 2.3, true is true, true is false, 'dog' is 'dog', 'dog' is 'cat', null is null, null is not null

1 is 1	2.3 is 2.3	true is true	true is false	'dog' is 'dog'	'dog' is 'cat'	null is null	null is not null
1	1	1	0	1	0	1	0