Fix assert error in FATP camera performance test

BUG=None
TEST=Manullay, run camera performance test with test_type=Full

Change-Id: If11385202a1b5e92725db9c5e6032f9e926c56b4
Reviewed-on: https://gerrit.chromium.org/gerrit/59006
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Queue: Justin Chuang <jchuang@chromium.org>
Tested-by: Justin Chuang <jchuang@chromium.org>
diff --git a/client/site_tests/factory_CameraPerformanceAls/factory_CameraPerformanceAls.py b/client/site_tests/factory_CameraPerformanceAls/factory_CameraPerformanceAls.py
index f1d71f2..821b672 100644
--- a/client/site_tests/factory_CameraPerformanceAls/factory_CameraPerformanceAls.py
+++ b/client/site_tests/factory_CameraPerformanceAls/factory_CameraPerformanceAls.py
@@ -508,6 +508,8 @@
                     self.log('No serial number detected: %s\n' % error_message)
             else:
                 self.serial_number = event.data.get('sn', '')
+        else:
+            self.serial_number = 'MISSING_SN'
 
         if self.type == _TEST_TYPE_FULL:
             with leds.Blinker(self._LED_RUNNING_TEST):