New snapshot

Create an empty commit so that we use the same snapshot file in external manifest-snapshot repo for R77.

Corresponding commit in internal manifest-snapshot is: https://cos-internal.googlesource.com/cos/manifest-snapshots-internal/+/e4792b07d2b96a8781626a5a93088b042e921e3f%5E%21/#F0
The internal manifest-snapshot commit is only updating the internal codebase and therefore, no change is needed for external manifest snapshot.

This is needed because internal and external manifest snapshot repository are using different git tags as buildnumber.

BUG=b/171912053

Change-Id: I75f2b7df67fb1c443bde5c1f49a6793a33c9872b