fix preview: empty -> blank

app/controllers/admin/users_controller.rb

app/controllers/extranet/account_controller.rb

app/controllers/users/registrations_controller.rb

app/models/communication/extranet/with_sso.rb

app/models/university/with_sso.rb

...ication/blocks/templates/call_to_action/_preview.html.erb

.../communication/blocks/templates/chapter/_preview.html.erb

.../communication/blocks/templates/contact/_preview.html.erb

...ommunication/blocks/templates/datatable/_preview.html.erb

...munication/blocks/templates/definitions/_preview.html.erb

...in/communication/blocks/templates/files/_preview.html.erb

.../communication/blocks/templates/gallery/_preview.html.erb

...in/communication/blocks/templates/image/_preview.html.erb

...munication/blocks/templates/key_figures/_preview.html.erb

...unication/blocks/templates/testimonials/_preview.html.erb

...communication/blocks/templates/timeline/_preview.html.erb

...in/communication/blocks/templates/video/_preview.html.erb

app/views/admin/research/laboratories/axes/show.html.erb