bugfix: sandbox_paths is nullable, check for it in AWS

[prandla]

jinja being too dynamic strikes again....

mostly this is a problem when the submission is still compiling, then both sandbox_paths and sandbox_digests are None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.67%. Comparing base (4cdaa00) to head (d4c62cf).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1632      +/-   ##
==========================================
- Coverage   54.67%   54.67%   -0.01%     
==========================================
  Files         335      335              
  Lines       27361    27361              
==========================================
- Hits        14960    14959       -1     
- Misses      12401    12402       +1     

Flag	Coverage Δ
functionaltests	0.00% <ø> (ø)
unittests	54.67% <ø> (-0.01%)	⬇️

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