sql: add tests for IS (NOT) NULL with empty tuples

Logic for `IS NULL` and `IS NOT NULL` with tuple operands was recently
fixed in cockroachdb#48299. This commit adds tests for empty tuples as operands
that were forgotten in that commit. This makes the correct evaluation
logic of such an expression explicit and will help prevent regressions.

Release note: None

pkg/sql/opt/norm/fold_constants_funcs.go

@@ -88,9 +88,10 @@ func (c *CustomFuncs) HasNullElement(input opt.ScalarExpr) bool {
 }
 
 // HasAllNullElements returns true if the input tuple has only constant, null
-// elements. Note that it only returns true if all elements are known to be
-// null. For example, given the tuple (NULL, x), it will return false because x
-// is not guaranteed to be null.
+// elements, or if the tuple is empty (has 0 elements). Note that it only
+// returns true if all elements are known to be null. For example, given the
+// tuple (NULL, x), it will return false because x is not guaranteed to be
+// null.
 func (c *CustomFuncs) HasAllNullElements(input opt.ScalarExpr) bool {
 	tup := input.(*memo.TupleExpr)
 	for _, e := range tup.Elems {
@@ -122,9 +123,10 @@ func (c *CustomFuncs) HasNonNullElement(input opt.ScalarExpr) bool {
 }
 
 // HasAllNonNullElements returns true if the input tuple has all constant,
-// non-null elements. Note that it only returns true if all elements are known
-// to be non-null. For example, given the tuple (1, x), it will return false
-// because x is not guaranteed to be non-null.
+// non-null elements, or if the tuple is empty (has 0 elements). Note that it
+// only returns true if all elements are known to be non-null. For example,
+// given the tuple (1, x), it will return false because x is not guaranteed to
+// be non-null.
 func (c *CustomFuncs) HasAllNonNullElements(input opt.ScalarExpr) bool {
 	tup := input.(*memo.TupleExpr)
 	for _, e := range tup.Elems {

pkg/sql/opt/norm/testdata/rules/comp

@@ -363,6 +363,16 @@ values
  ├── fd: ()-->(1)
  └── (true,)
 
+norm expect=FoldNullTupleIsTupleNull
+SELECT () IS NULL AS r
+----
+values
+ ├── columns: r:1!null
+ ├── cardinality: [1 - 1]
+ ├── key: ()
+ ├── fd: ()-->(1)
+ └── (true,)
+
 norm expect-not=FoldNullTupleIsTupleNull
 SELECT (k, NULL) IS NULL FROM a
 ----
@@ -535,6 +545,16 @@ values
  ├── fd: ()-->(1)
  └── (true,)
 
+norm expect=FoldNonNullTupleIsTupleNotNull
+SELECT () IS NOT NULL AS r
+----
+values
+ ├── columns: r:1!null
+ ├── cardinality: [1 - 1]
+ ├── key: ()
+ ├── fd: ()-->(1)
+ └── (true,)
+
 norm expect-not=FoldNonNullTupleIsTupleNotNull
 SELECT (1, k) IS NOT NULL FROM a
 ----

pkg/sql/sem/tree/testdata/eval/is

@@ -524,3 +524,15 @@ eval
 NOT ((NULL, NULL) IS DISTINCT FROM NULL)
 ----
 false
+
+# Empty tuples.
+
+eval
+() IS NULL
+----
+true
+
+eval
+() IS NOT NULL
+----
+true