String for blank security "user credentials" page

This is shown when the user doesn't have any private keys in TPM yet.

Bug: 30878596
Change-Id: Ic24acf4b5251ec442babfddd4d4212e05c461ec4
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 0adda7b..760f652 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -5560,6 +5560,8 @@
     <string name="user_credential_title">Credential Details</string>
     <!-- Announcement to confirm a user credential being removed. [CHAR LIMIT=NONE] -->
     <string name="user_credential_removed">Removed credential: <xliff:g id="credential_name" example="signing key">%s</xliff:g></string>
+    <!-- Placeholder for the list of installed user credentials (private keys) when the list is empty. [CHAR LIMIT=120] -->
+    <string name="user_credential_none_installed">No user credentials installed</string>
 
     <!--  Title for spell checker settings -->
     <string name="spellcheckers_settings_title">Spell checker</string>