This PR is extracted from the sqlite fuzzing exploration effort in
https://github.com/tursodatabase/limbo/pull/1021
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We were not evaluating constant conditions (e.g '1 IS NULL') when there
were no tables referenced in the query, because our WHERE term
evaluation was based on "during which loop" to evaluate them. However,
when there are no tables, there are no loops, so they were never
evaluated.
Closes#1023