Can't submit a new benchmark without JSON schema

[vykozlov]

Describe the bug
Trying to submit a new benchmark without JSON schema (which should be optional)

To Reproduce
Steps to reproduce the behavior:

1. https://perf.test.fedcloud.eu/benchmark-submission
2. fill the fields, in the "JSON template" put nothing, as this field should be optional
3. See error
   { "code": 422, "errors": { "json": { "json_schema": [ "Missing data for required field." ] } }, "status": "Unprocessable Entity" }

Expected behavior
A new Benchmark is added

[TheChristophe]

The frontend submits no json_schema field if the text box is left empty, but backend always expects something. There are 2 possible solutions:

• Allow nullable json_schema to represent "no schema submitted"
• Instead, use the empty schema "{}" and default to that in frontend or backend and have "{}" also mean "no schema submitted"

[BorjaEst]

From our last internal meeting:

• Requires to have a schema, introduces less errors and less "bad" data.
• {} should be also forbiden (from backend)