fix json_extract panic

Author: James Cor

enginetest/queries/queries.go

@@ -9854,6 +9854,12 @@ from typestable`,
 			{"NULL"},
 		},
 	},
+	{
+		Query: `SELECT json_type(json_extract('{"a": null}', null));`,
+		Expected: []sql.Row{
+			{"NULL"},
+		},
+	},
 	{
 		Query: "SELECT json_type(cast(cast('2001-01-01 12:34:56.123456' as datetime) as json));",
 		Expected: []sql.Row{

sql/expression/function/json/json_extract.go

@@ -101,6 +101,10 @@ func (j *JSONExtract) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
 		if err != nil {
 			return nil, err
 		}
+		
+		if path == nil {
+			continue
+		}
 
 		results[i], err = types.LookupJSONValue(searchable, path.(string))
 		if err != nil {