Your relay outputs an empty channel, which is different to "no channel". There is no need to use a relay, simply use a case structure that outputs some meaningful text if a channel is inactive. (this also avoids all the useless signal processing of the disabled channel).
It is important to leave something in place (it could even be an empty string array element), otherwise your higher active channels would move down a slot if a lower channel is disabled.
The attached example shows one possibility, you can easily expand it to 8 channels. Modify as needed.
🙂