Fix DeltaPerformerIntegrationTest crash.

Got this error running unittest in Chrome OS.
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid

MountFilesystem() expects a const string& in the 5th parameter, will crash
if nullptr is passed.

Bug: None
Test: cros_workon_make update_engine --test

Change-Id: I14d07e6c60c39ce9604f69613fba584723c598c5
diff --git a/common/test_utils.cc b/common/test_utils.cc
index 77a9141..f4d203c 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -260,7 +260,7 @@
   string loop_dev;
   loop_binder_.reset(new ScopedLoopbackDeviceBinder(file_path, &loop_dev));
 
-  EXPECT_TRUE(utils::MountFilesystem(loop_dev, *mnt_path, flags, "", nullptr));
+  EXPECT_TRUE(utils::MountFilesystem(loop_dev, *mnt_path, flags, "", ""));
   unmounter_.reset(new ScopedFilesystemUnmounter(*mnt_path));
 }