MDEV-11438 Assertion `null_value' failed in Item::send(Protocol*, String*) upon casting NULL as JSON.

        test added.

Author: Alexey Botchkov

mysql-test/r/func_json.result

@@ -244,6 +244,9 @@ json_object("a", '{"b": "abcd"}')
 select json_object("a", cast('{"b": "abcd"}' as json));
 json_object("a", cast('{"b": "abcd"}' as json))
 {"a": {"b": "abcd"}}
+select cast(NULL AS JSON);
+cast(NULL AS JSON)
+NULL
 select json_depth(cast(NULL as JSON));
 json_depth(cast(NULL as JSON))
 NULL

mysql-test/t/func_json.test

@@ -108,5 +108,6 @@ select json_object("a", json_object("b", "abcd"));
 select json_object("a", '{"b": "abcd"}');
 select json_object("a", cast('{"b": "abcd"}' as json));
 
+select cast(NULL AS JSON);
 select json_depth(cast(NULL as JSON));