odb: `git_odb_object` contents are never NULL

@carlosmn's commit at https://github.com/libgit2/libgit2/commit/e1ac0101480c29631ad56409d77f2dd7b65bfd09 breaks the contract that the contents of a blob are never `NULL` even if the blob is empty.

This breaks (amongst other things) the attributes and filter code as soon as empty blobs are involved. Don't do that!