sql/json: fix null array support in inverted index support

We assumed all the array elements were strings w/o checking for null,
now we use AsDString to check it.

Found internally with expanded sqlsmith testing.

Fixes: #101025
Epic: None
Release note: None

pkg/sql/logictest/testdata/logic_test/json

@@ -607,6 +607,15 @@ SELECT true FROM x WHERE j->'a' @> '2'::JSONB
 ----
 true
 
+query T
+SELECT j FROM x WHERE j ?| ARRAY[NULL]
+----
+
+query T
+SELECT j FROM x WHERE j ?& ARRAY[NULL::STRING]
+----
+{"a": [1, 2, 3]}
+
 query T
 SELECT '{"foo": {"bar": 1}}'::JSONB #- ARRAY['foo', 'bar']
 ----

pkg/sql/rowenc/index_encoding.go

@@ -704,8 +704,11 @@ func EncodeExistsInvertedIndexSpans(
 		}
 		var expr inverted.Expression
 		for _, d := range val.(*tree.DArray).Array {
-			s := string(*d.(*tree.DString))
-			newExpr, err := json.EncodeExistsInvertedIndexSpans(nil /* inKey */, s)
+			ds, ok := tree.AsDString(d)
+			if !ok {
+				continue
+			}
+			newExpr, err := json.EncodeExistsInvertedIndexSpans(nil /* inKey */, string(ds))
 			if err != nil {
 				return nil, err
 			}