sql: fix ColumnAccessExpr.Eval with NULL inner expression

Previously, `ColumnAccessExpr.Eval` would panic if the
`ColumnAccessExpr`'s inner expression evaluated to `NULL`, because it
attempted to cast this `NULL` to a `DTuple`. Now, if the inner
expression is `NULL`, `ColumnAccessExpr.Eval` returns `NULL`.

Fixes #78159

Release note (bug fix): A bug has been fixed that caused an internal
error when the inner expression of a column access expression evaluated
to `NULL`. For example, evaluation of the expression
`(CASE WHEN b THEN ((ROW(1) AS a)) ELSE NULL END).a` would error when
`b` is `false`. This bug has been present since version 19.1 or earlier.

pkg/sql/logictest/testdata/logic_test/tuple

@@ -978,3 +978,18 @@ CREATE TABLE t74729 AS SELECT g % 2 = 1 AS _bool FROM generate_series(1, 5) AS g
 
 statement ok
 SELECT CASE WHEN _bool THEN (1, ('a', 2)) ELSE (3, NULL) END FROM t74729
+
+# Regression test for #78159. Column access of NULL should not cause an internal
+# error.
+subtest 78159
+
+statement ok
+CREATE TABLE t78159 (b BOOL)
+
+statement ok
+INSERT INTO t78159 VALUES (false)
+
+query B
+SELECT (CASE WHEN b THEN ((ROW(1) AS a)) ELSE NULL END).a from t78159
+----
+NULL

pkg/sql/sem/tree/eval.go

@@ -3927,6 +3927,9 @@ func (expr *ColumnAccessExpr) Eval(ctx *EvalContext) (Datum, error) {
 	if err != nil {
 		return nil, err
 	}
+	if d == DNull {
+		return d, nil
+	}
 	return d.(*DTuple).D[expr.ColIndex], nil
 }