Add empty state to Deploy Keys admin section

- add empty state partial inspired by GitLab UI empty state regions
- harmonize action button appereance

app/views/admin/deploy_keys/index.html.haml

 - page_title _('Deploy Keys')
-
-%h3.page-title.deploy-keys-title
-  = _('Public deploy keys (%{deploy_keys_count})') % { deploy_keys_count: @deploy_keys.load.size }
-  .float-right
-    = link_to _('New deploy key'), new_admin_deploy_key_path, class: 'btn btn-success btn-sm btn-inverted'
-
 - if @deploy_keys.any?
+  %h3.page-title.deploy-keys-title
+    = _('Public deploy keys (%{deploy_keys_count})') % { deploy_keys_count: @deploy_keys.load.size }
+    = link_to _('New deploy key'), new_admin_deploy_key_path, class: 'float-right btn btn-success btn-md gl-button'
   .table-holder.deploy-keys-list
     %table.table
       %thead
@@ -32,3 +29,5 @@
               .float-right
                 = link_to _('Edit'), edit_admin_deploy_key_path(deploy_key), class: 'btn btn-sm'
                 = link_to _('Remove'), admin_deploy_key_path(deploy_key), data: { confirm: _('Are you sure?') }, method: :delete, class: 'btn btn-sm btn-remove delete-key'
+- else
+  = render 'shared/empty_states/deploy_keys'

app/views/shared/empty_states/_deploy_keys.html.haml

+.empty-state.gl-display-flex.gl-flex-direction-column.gl-flex-wrap.gl-text-center
+  .gl-flex-grow-0.gl-flex-shrink-0
+    .svg-250.svg-content
+      = image_tag 'illustrations/empty-state/empty-deploy-keys-lg.svg'
+  .gl-flex-grow-0.gl-flex-shrink-0
+    .text-content.gl-mx-auto.gl-my-0.gl-p-5
+      %h4.h4= _('Deploy keys allow read-only or read-write (if enabled) access to your repository')
+      %p= _('Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one.')
+      = link_to _('New deploy key'), new_admin_deploy_key_path, class: 'btn btn-success btn-md gl-button'

ee/changelogs/unreleased/36349-add-missing-empty-state-for-deploy-keys.yml

+---
+title: Add empty state for Deploy Keys section
+merge_request: 38329
+author:
+type: changed

locale/gitlab.pot

@@ -7910,9 +7910,15 @@ msgstr ""
 msgid "Deploy key was successfully updated."
 msgstr ""
 
+msgid "Deploy keys allow read-only or read-write (if enabled) access to your repository"
+msgstr ""
+
 msgid "Deploy keys allow read-only or read-write (if enabled) access to your repository. Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one."
 msgstr ""
 
+msgid "Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one."
+msgstr ""
+
 msgid "Deploy progress not found. To see pods, ensure your environment matches %{linkStart}deploy board criteria%{linkEnd}."
 msgstr ""