Don't notify when 'Slack Channel' is empty.

If 'Slack Channel' project custom field empty, current code will get channel from `Setting.plugin_redmine_slack[:channel]`. In some cases, the plugin default channel is kept as empty to avoid sending notification to default channel. However, current implementation get `''` from `Setting.plugin_redmine_slack[:channel]` and `channel=''` lead a notification.

Workaround: set `'<not-existent-channel-name>'` to plugin default channel value.