adb: fix test_device.FileOperationsTest.test_push_empty

Test: python test_device.py
Change-Id: Iaa5006e39c3d61374de11c05bcaffb4f30b7512e
diff --git a/adb/test_device.py b/adb/test_device.py
index 4abe7a7..c8a4d9b 100644
--- a/adb/test_device.py
+++ b/adb/test_device.py
@@ -762,9 +762,10 @@
             os.chmod(host_dir, 0o700)
 
             # Create an empty directory.
-            os.mkdir(os.path.join(host_dir, 'empty'))
+            empty_dir_path = os.path.join(host_dir, 'empty')
+            os.mkdir(empty_dir_path);
 
-            self.device.push(host_dir, self.DEVICE_TEMP_DIR)
+            self.device.push(empty_dir_path, self.DEVICE_TEMP_DIR)
 
             test_empty_cmd = ['[', '-d',
                               os.path.join(self.DEVICE_TEMP_DIR, 'empty')]