Cater for key not found

[tpmccallum]

When the C++ calls this library it provides a DB key to the value.
If the key is found in the DB this library returns a pointer to the data and the length

A solution is required for cases where the key does not exist. For example, C++ needs to get back something meaningful (using the same data types) which allows C++ to determine that the key does not exist.

Perhaps the get_byte_array_length function could return 0