Contact Groups: Prevent users from trying to create a contact group, when there are no contacts

[flourish86]

In #276 we address the issue, when no Channels are available to select.

Then there's a similar situation for contact groups. When no contacts are created, it will be impossible to create a new Contact Group in the first place.

We should adapt the solution for this scenario:

• Add a hint within the empty state
• link to the contact creation tab accordingly
• Disable the "Add contact group" button

Users hit a dead end, when they want to create a a contact group, but no contacts are available to select