Deprecate empty user identifier

Author: ajgarlag

CHANGELOG.md

@@ -6,6 +6,7 @@ CHANGELOG
 
  * Add `$token` argument to `UserCheckerInterface::checkPostAuth()`
  * Deprecate argument `$secret` of `RememberMeToken`
+ * Deprecate returning an empty string in `UserInterface::getUserIdentifier()`
 
 7.0
 ---

User/UserInterface.php

@@ -56,6 +56,8 @@ public function eraseCredentials(): void;
 
     /**
      * Returns the identifier for this user (e.g. username or email address).
+     *
+     * @return non-empty-string
      */
     public function getUserIdentifier(): string;
 }