Zabbix server accepts timeout error string "timeout while executing a shell script" as a good result. This phenomena occurs under using system.uname, system.hostname, system.run and UserParameter with data type stirng.
The source of the bug is following.
1. in zbx_execute(), if timeout occurs, "timeout while executing a shell script" string is set to 'error' variable.
2. in EXECUTE_STR(), which calls zbx_execute(), copies the string in error variable to result variable.
3. Zabbix Agent resturns the string in result.
4. Zabbix Server get the string and it match items' data type.

In my research, upper 4 items are influenced. EXECUTE_STR() should not copy and return empty string. By such a way, Zabbix Server get the item NOTSUPPORTED as other poller behaves.

In addition, "timeout while executing a shell script" is printed to the log file in zbx_execute(). So, there is no problem to return empty string.