Passing wrong payload to sc-server's `create_elements_by_scs` causes a crash

[FallenChromium]

Describe the bug
After sending an empty payload to create_elements_by_scs method of SC-JSON API provided by sc-server it crashed with SIGABRT.

To Reproduce
Steps to reproduce the behavior:

1. Start sc-server
2. Establish a websocket connection
3. Send empty payload to create_elements_by_scs endpoint

Expected behavior
I am not sure if it should raise an error or pass it as a valid construction, but it definitely shouldn't stop execution.

Screenshots

Desktop (please complete the following information):

• OS: Ubuntu 20.04 in Docker
• Version 0.7.0

[NikitaZotov]

Fixed in #137