Allow ExtractToMemory to accept an empty buffer for empty entries

We don't actually need to extract the empty entries. Since the old
code support extracting the empty entry to a empty buffer, add the
support back in ExtractToMemory.

Bug: 153393683
Test: unittests pass

Change-Id: Idb9f0f4e6e4ffd4b44b80ddd3f54069bb7cedd7b