Check that CommitLogManifest exists before deletion

It appears that there are some possible flows where the reducer runs twice on
the same key. Either because of some error in a subsequent key that makes the
entire shard become ignored and retried, or possible some obscure error outside
of the transaction on that key.

The result however is that sometimes the reducer runs on a key that has already
been deleted. We need to check for that to prevent a null pointer exception.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166112173

java/google/registry/backup/BUILD

@@ -20,6 +20,7 @@ java_library(
         "@com_google_appengine_api_1_0_sdk",
         "@com_google_appengine_tools_appengine_gcs_client",
         "@com_google_appengine_tools_appengine_mapreduce",
+        "@com_google_auto_value",
         "@com_google_code_findbugs_jsr305",
         "@com_google_dagger",
         "@com_google_guava",