Turn permission error into validation error for empty streams in event definition.

[dennisoelkers]

Note: This is based on #22771, which needs to be merged before.

Description

Motivation and Context

This PR is making sure that if a user is creating an event definition, does not have the global streams:read permission and has an empty streams field in the event definition config, a validation error will be returned instead of a 403. This way, the frontend can handle it and show the user a helpful message in the event definition creation dialog instead of ending up on an error page.

Fixes Graylog2/graylog-plugin-enterprise#10869.

/prd https://github.com/Graylog2/graylog-plugin-enterprise/pull/10970

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.