Description for index sets should be optional

## Expected Behavior

Similar to streams, I can create an index set without having to specify a description.

## Current Behavior

Description is mandatory and not specifying it will block index set creation.

## Context

## Your Environment

* Graylog Version:
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version:

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?

Start by tracing index set creation and compare its description validation with the similar stream flow. Confirm where the mandatory-description check is applied and whether the request and UI both enforce it. Done means an index set can be created without a description, while descriptions that are provided continue to work.

Written by the indexing model from the issue text.