Show appropriate message for View button when document can not be viewed in any channel

The View button in the document editor can list one or more channels in which the document can be (pre)viewed. Sometimes there is no preview available (can be due to several reasons). In that case an very small and empty dropdown is shown. This looks buggy and is not a very nice effect.

Suggested improvement: show a message there that informs the editor about the situation. E.g. "No channel available to preview this document".

Please do not hide the button in this case! Editor must not look for missing button, but be informed about the situation.