UI: Fix Auto-Config Wizard for custom server with no stream key

The Auto-Configuration Wizard cannot progress when selecting a custom
service that does not require a stream key. Since we have no way to
validate if a custom service requires a stream key, we should just not
require a stream key is custom service is selected. To fix this, check
if a custom service has not been selected when checking if the stream
key is empty.

Co-authored-by: Ryan Foster <ryan@obsproject.com>
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