odb: `git_odb_object` contents are never NULL

This is a contract that we made in the library and that we need to uphold. The
contents of a blob can never be NULL because several parts of the library (including
the filter and attributes code) expect `git_blob_rawcontent` to always return a
valid pointer.
Merge pull request #2724 from libgit2/vmg/empty

odb: `git_odb_object` contents are never NULL