libsnapshot_fuzzer: Add ZERO to operation types

Even though operation type ZERO is not used
in libsnapshot, add it so that the fuzzer has
an alternative operation type to use. Without
it, the fuzzer will only generate operations
SOURCE_COPY.

Test: pass
Bug: 154633114
Change-Id: I4fd36421b8b33ed68b94be2d739a6dd706b3829f