Improvements to "NOT IN (SELECT ...)" processing. Only test for NULL values

Improvements to "NOT IN (SELECT ...)" processing.  Only test for NULL values
on the RHS on the first iteration, then remember the result.  There has been
logic to do this for year, but it didn't work right and ended up repeating
the NULL test on every iteration.  This inefficiency was found using the
VDBE coverage testing tools.

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