fix: allow None for type Optional[conset / conlist]

[PrettyWood]

Change Summary

We were not handling the None case in set_length_validator causing issues when Optional[conset(...)] type had a set value equal to None.
Furthermore the check on required for conlist was wrong as Optional means "nullable" and we can have required nullables

Related issue number

closes #2320

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2321 (84afe71) into master (13a5c7d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2321   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         4199      4201    +2     
  Branches       854       855    +1     
=========================================
+ Hits          4199      4201    +2     

Impacted Files	Coverage Δ
pydantic/types.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13a5c7d...84afe71. Read the comment docs.

[samuelcolvin]

thanks a lot.