Steps to reproduce:

1. Changes in configuration...
   1. Enable a web scenarios with a step. In this step, configure "Raw post", Headers, end leave "Required string" empty.

Result:
The zabbix-server crash immediately, see the log in the attached file.

As mentionned in the documentation, "If empty, no check on required string is performed". And in my case, i would like check only a HTTP response code because its an API with no output, only headers.

If you fill only one character in "Required string" the server will not crash.