protobuf: Server should gracefully handle non-file entities with file-like attributes.

Example C++ code:

Entity rt;
rt.SetRole(Role::RECORD_TYPE);
rt.SetName("Not a FILE");
rt.SetFilePath("");
insert_transaction->InsertEntity(&rt);
insert_transaction->Execute();

DoD

server bug was fixed with !53 (merged)

• review and merge caosdb-cpplib!34 (merged) which is a dependency for the next
• review and merge caosdb-cppinttest!19 (merged) which confirms that this issues is fixed.