Fix split focusing being broken in certain circumstances when the "Show input when it's empty" setting was disabled (#3838)

Co-authored-by: Kasia <zneix@zneix.eu>

src/common/Channel.cpp

@@ -336,7 +336,7 @@ IndirectChannel::IndirectChannel(ChannelPtr channel, Channel::Type type)
 {
 }
 
-ChannelPtr IndirectChannel::get()
+ChannelPtr IndirectChannel::get() const
 {
     return data_->channel;
 }