Lint: Fix pyright errors in s3 module

[ByteOtter]

What this PR does / why we need it:

Fixes mainly typing issues in the s3 module, mainly by explicitly casting or typing as Noneable.

Which issue(s) this PR fixes:
Tracks #152

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.03%. Comparing base (ef7d368) to head (dc3bd9f).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/s3/bucket.py	90.00%	1 Missing ⚠️
src/gardenlinux/s3/s3_artifacts.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
- Coverage   93.22%   89.03%   -4.20%     
==========================================
  Files          30       33       +3     
  Lines        1372     1495     +123     
==========================================
+ Hits         1279     1331      +52     
- Misses         93      164      +71     

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