Fix empty CTCP ACTION sending two literal underscores on Discord

[yutotakano]

The Bug that this PR Fixes

Sending an empty CTCP ACTION (i.e. a /me command) from IRC will make go-discord-irc send the message __ (two underscores) to Discord. This renders on Discord clients as literal underscores instead of an empty message, breaking the expectation of the IRC user and changing the perceived content of the message.

Having any non-zero whitespace in the CTCP ACTION will not cause this issue (only the true empty string), as Discord renders the sequence _ _ as empty but __ as two literal underscores.

This issue is rather obscure since most IRC clients forbid you from sending an empty CTCP ACTION. However, with the use of programmatic bots/clients, you can do this. (It is common when you want to play with the bot's nickname). The intended look of these messages is that it shows the nick of the user in emphasis, without a message to follow it. So, it is a bug in go-discord-irc that there is a visible message content.

The Implemented Fix

The wrapping of IRC CTCP ACTION commands with underscores to mimic the italicization on Discord happens before the logic for ZWB treatment of empty messages. This means that we wrap the empty CTCP ACTION with underscore characters, before sending it onto the Go channel read by bridge.go, which then sees a non-empty message and thus does not insert ZWBs.

This commit fixes the bug by not wrapping a CTCP ACTION with underscores if the content is empty. This way, an empty message is sent onto the Go channel read by bridge.go, which then will give the necessary ZWB treatment, and send an empty message onto Discord.

[yutotakano]

Not very sure what the CI failure is caused by, but this issue in golangci-lint says that a version bump should fix it.