Fix TypeError when submitting w/o SLACK_CHANNELS config
Fix TypeError when submitting w/o SLACK_CHANNELS config

Introduced with the merge of https://github.com/rauchg/slackin/pull/152/,
the problem was if the `SLACK_CHANNELS` environment variable wasn't set
up, then no hidden form element with the listed channels would be
included. Therefore, our `channel` variable would be `undefined`, and a
direct access of the `value` property would crash the app.

Ineeza/luka_ineeza_slakin,thanosIrodotou/slackin,ouluwebdev/slackin,TibiaPF/slackin,Code-T/slackin,devopsnz/slackin,emccode/community,thanosIrodotou/slackin,emccode/community,edorgeville/slackin-fr,Code-T/slackin,IOT-DSA/slackin,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,avaclancheuk/slackin,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,rauchg/slackin,ouluwebdev/slackin,emccode/community,avaclancheuk/slackin,lemonlabsuk/slack-invite-page,TechnologyMasters/technologymasters.herokuapp.com,edorgeville/slackin-fr,szkl/slackin,Ineeza/luka_ineeza_slakin,mike1pol/slackin,Venpath/slackin,thanosIrodotou/slackin,mike1pol/slackin,mbbroberg/slackin,szkl/slackin,rauchg/slackin,lemonlabsuk/slack-invite-page,devopsnz/slackin,TibiaPF/slackin,TechnologyMasters/technologymasters.herokuapp.com,mbbroberg/slackin,IOT-DSA/slackin,Venpath/slackin