Change the return type to a std::pair<bool, QPDFObjectHandle> in order to
allow a default constructed object handle (which is currently returned to
indicate failure) to become a valid object.
Create unresolved objects only for objects in the xref table (except during
parsing of the xref table). Do not add indirect nulls into the the object
cache as the result of a cache miss during a call to getObject except
during parsing or creation/updating from JSON. To support this behaviour,
add new private methods getObjectForParser and getObjectForJSON.
As a result of this change, dangling references are treated as direct nulls
rather than indirect nulls.
