Prevent create channel with users without username

packages/rocketchat-lib/server/models/Users.coffee

...es/rocketchat-ui-flextab/flex-tab/tabs/membersList.coffee

...s/rocketchat-ui-sidenav/side-nav/createChannelFlex.coffee

.../rocketchat-ui-sidenav/side-nav/createCombinedFlex.coffee

.../rocketchat-ui-sidenav/side-nav/directMessagesFlex.coffee

...s/rocketchat-ui-sidenav/side-nav/privateGroupsFlex.coffee