allow NULL empty blocks in blk2bstr

[rdmark]

Treat a NULL source pointer as valid when len is zero, since no source memory is read and the result is an ordinary empty bstring. Keep negative lengths and NULL pointers paired with a positive length as errors.

Document the contract in the public header, and cover the accepted form with the core unit test.

Thanks to @DamjanJovanovic for the report

[github-actions]

File	Coverage	Lines	Branches
All files	69%	74%	63%	✅
bstring/bstraux.c	55%	64%	47%	✅
bstring/bstrlib.c	74%	78%	71%	✅
bstring/buniutil.c	80%	86%	73%	✅
bstring/utf8util.c	60%	71%	50%	✅

Minimum allowed coverage is 50%

Generated by 🐒 cobertura-action against 85a5247

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud