handle calling getUser in test (#22833)

Author: lfolco

app/code/Magento/Security/Model/AdminSessionsManager.php

@@ -154,6 +154,7 @@ public function processProlong()
             $this->getCurrentSession()->save();
         }
 
+        // todo: don't necessarily have a user here
         if ($this->authSession->getUser()->getExpiresAt())
         {
             $this->revokeExpiredAdminUser();

app/code/Magento/Security/Test/Unit/Model/AdminSessionsManagerTest.php

@@ -253,6 +253,10 @@ public function testProcessProlong()
             ->method('save')
             ->willReturnSelf();
 
+        $this->authSessionMock->expects($this->once())
+            ->method('getUser')
+            ->willReturn($this->userMock);
+
         $this->model->processProlong();
     }