if there's no pending invitations, display nothing (i.e. no header, table, followup paragraph), like

```
- unless @group.pending_invitations.count == 0
  %h1=  t :pending_invitations
  %table.pending-invitations.table.table-striped
    %thead
      %th= t "common.email"
      %th= t "invitation.invitation_link"
    %tbody
      - @group.pending_invitations.each do |invitation|
        %tr
          %td= invitation.recipient_email
          %td= invitation_url(invitation)
          %td= link_to(t(:cancel), group_invitation_path(@group, invitation), method: :delete, class: 'btn')

  %p= t "invitation.copy"
```