cherrypick-1.1: sql: fix tuple equality comparisons for tuples with NULL values

[RaduBerinde]

Cherry-picking the fixes in #21115 and #21230. CC @cockroachdb/release

sql: fix tuple equality comparisons for tuples with NULL values

Fixes #21113.

Previously, tuple equality evaluation would short-circuit on NULL elements
just like tuple inequality evaluation. This behavior was desired for tuple
comparisons using inequality operators but was a deviation from the SQL
standard for tuple comparisons using the equality operator.

In other words, (1, 2, 4) > (1, NULL, 5) evaluated to NULL, correctly,
but (1, 2, 4) = (1, NULL, 5) also evaluated to NULL, incorrectly. Instead,
the latter expression should have evaluated to false. This is because tuple
comparison should only return NULL when the non-NULL elements are not sufficient
to determine the result. Since tuple inequality is defined lexicographically,
the first NULL element encountered causes ambiguity. For tuple equality, it
may still be possible to evaluate the expression even after NULL elements are
seen, so the evaluation cannot short circuit.

This change fixes the behavior for tuple equality, bringing it inline with
PostgreSQL and MySQL.

Note that we already had very similar logic for the IN, ANY, SOME, and
ALL comparison operators, so the behavior replaced here must have simply
been an oversight when tuple comparison was introduced.

Release note (sql change/bug fix): Fix tuple equality to evaluate correctly
in the presence of NULL elements.

sql: fix not-NULL spans for tuple !=

PR #21115 fixed the logic for tuple != tuple which was incorrectly handling
NULLs. This change fixes the corresponding logic in the index selection code
(which is incorrectly generating /!NULL spans).

Release note (sql change/bug fix): Fix tuple equality to evaluate correctly
in the presence of NULL elements.

[cockroach-teamcity]

This change is