Fix objects that can have a NULL object id as attribute

[GwnDaan]

There are attributes for some of the objects that are allowed to be the NULL object id. This PR wil fix working with pools that have those id's set to NULL.

[codecov]

Codecov Report

Attention: 167 lines in your changes are missing coverage. Please review.

Comparison is base (da74b03) 11.52% compared to head (b40a528) 10.68%.

Files	Patch %	Lines
src/object_pool/object.rs	0.00%	102 Missing ⚠️
src/object_pool/reader.rs	0.00%	45 Missing ⚠️
src/object_pool/writer.rs	0.00%	10 Missing ⚠️
src/object_pool/object_pool.rs	0.00%	6 Missing ⚠️
src/object_pool/object_id.rs	0.00%	3 Missing ⚠️
src/object_pool/vt_version.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   11.52%   10.68%   -0.84%     
==========================================
  Files          17       17              
  Lines        2664     2723      +59     
==========================================
- Hits          307      291      -16     
- Misses       2357     2432      +75     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.