MDEV-37428 JSON_VALUE returns NULL for a key with an empty string value rather than an empty string

Regression from MDEV-36765 / 2b24ed8.

json_unescape can return a string length 0 without it being an error.

The regression caused this 0 length empty string to appear as an
error and result in a NULL return value.

Author: grooverdan

mysql-test/main/func_json.result

@@ -2686,4 +2686,10 @@ json_detailed('[[123],456]')
     [123],
     456
 ]
+#
+# MDEV-37428 JSON_VALUE returns NULL for a key with an empty string value rather than an empty string
+#
+SELECT JSON_VALUE(JSON_OBJECT("a", ""), '$.a') = "" AS not_null;
+not_null
+1
 # End of 10.11 Test

mysql-test/main/func_json.test

@@ -1947,4 +1947,10 @@ select json_detailed('[[123],456]');
 set @@collation_connection=@save_collation_connection;
 select json_detailed('[[123],456]');
 
+--echo #
+--echo # MDEV-37428 JSON_VALUE returns NULL for a key with an empty string value rather than an empty string
+--echo #
+
+SELECT JSON_VALUE(JSON_OBJECT("a", ""), '$.a') = "" AS not_null;
+
 --echo # End of 10.11 Test

sql/item_jsonfunc.cc

@@ -109,12 +109,14 @@ bool st_append_json(String *s,
     return false;
   }
 
-  if ((str_len= json_unescape(json_cs, js, js + js_len,
-         s->charset(), (uchar *) s->end(), (uchar *) s->end() + str_len)) > 0)
-  {
+  str_len= json_unescape(json_cs, js, js + js_len, s->charset(),
+                         (uchar *) s->end(), (uchar *) s->end() + str_len);
+  if (str_len > 0)
     s->length(s->length() + str_len);
+
+  if (str_len >= 0)
     return false;
-  }
+
   if (current_thd)
   {
     if (str_len == JSON_ERROR_OUT_OF_SPACE)