MDEV-12262 Assertion `!null_value' failed in virtual bool Item::send on

JSON_REMOVE.

        null_value wasn't properly set in Item_json_func_remove::val_str.

mysql-test/r/func_json.result

@@ -618,3 +618,14 @@ connection default;
 set @@global.max_allowed_packet = default;
 set @@global.net_buffer_length = default;
 disconnect newconn;
+create table t1(j longtext, p longtext);
+insert into t1 values
+('{"a":1,"b":2,"c":3}','$.a'),
+('{"a":1,"b":2,"c":3}','$.b'),
+('{"a":1,"b":2,"c":3}','$.c');
+select j, p, json_remove(j, p) from t1;
+j	p	json_remove(j, p)
+{"a":1,"b":2,"c":3}	$.a	{"b": 2, "c": 3}
+{"a":1,"b":2,"c":3}	$.b	{"a": 1, "c": 3}
+{"a":1,"b":2,"c":3}	$.c	{"a": 1, "b": 2}
+drop table t1;

mysql-test/t/func_json.test

@@ -263,3 +263,14 @@ set @@global.max_allowed_packet = default;
 set @@global.net_buffer_length = default;
 --disconnect newconn
 
+
+#
+# MDEV-12262 Assertion `!null_value' failed in virtual bool Item::send on JSON_REMOVE.
+#
+create table t1(j longtext, p longtext);
+insert into t1 values
+    ('{"a":1,"b":2,"c":3}','$.a'),
+    ('{"a":1,"b":2,"c":3}','$.b'),
+      ('{"a":1,"b":2,"c":3}','$.c');
+select j, p, json_remove(j, p) from t1;
+drop table t1;

sql/item_jsonfunc.cc

@@ -2633,6 +2633,7 @@ String *Item_func_json_remove::val_str(String *str)
     str->length(0);
 
     if (append_simple(str, js->ptr(), rem_start - js->ptr()) ||
+        (je.state == JST_KEY && n_item > 0 && str->append(",", 1)) ||
         append_simple(str, rem_end, js->end() - rem_end))
           goto js_error; /* Out of memory. */
 
@@ -2658,6 +2659,7 @@ String *Item_func_json_remove::val_str(String *str)
   if (json_nice(&je, str, Item_func_json_format::LOOSE))
     goto js_error;
 
+  null_value= 0;
   return str;
 
 js_error: