Fix 500 errors in Projects::AutocompleteSourcesController

What does this MR do and why?

This commit fixes 500 errors when:

1. No type parameter specified for commands, members, or labels. Now we return 400 Bad Request.
2. When an anonymous user attempts to load the list of commands for a public project. Previously this would error out while checking for permissions; now the endpoint returns an empty array.

Relates to gitlab-com/gl-infra/production#7820 (closed) #376088 (closed)

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