header_cta.external: handle blank correctly

app/views/admin/communication/websites/pages/static.html.erb

@@ -45,6 +45,6 @@ header_cta:
   label: >-
     <%= prepare_text_for_static @l10n.header_cta_label %>
   target: "<%= @l10n.header_cta_url %>"
-  external: <%= !(@l10n.header_cta_url.to_s.start_with?('/') || @l10n.header_cta_url.to_s.start_with?(@website.url)) %>
+  external: <%= @l10n.header_cta_url.present? && !(@l10n.header_cta_url.start_with?('/') || @l10n.header_cta_url.start_with?(@website.url)) %>
 <%= render 'admin/communication/contents/static', about: @l10n %>
 ---