fix trying to iterate over NoneType

[jacklanchantin]

Description

Sometimes there are no items in the world spec. Unless that itself is a bug, this fixes the bug of trying to iterate over items of type None.

Fixes # (issue)

Type of change

Please check the options that are relevant.

• Bug fix (non-breaking change that fixes an issue)
• Proposes a change (non-breaking change that isn't necessarily a bug)
• Refactor
• New feature (non-breaking change that adds a new functionality)
• Breaking change (fix or feature that would break some existing functionality downstream)
• This is a unit test
• Documentation only change
• Datasets Release
• Models Release

Type of requested review

• I want a thorough review of the implementation.
• I want a high level review.
• I want a deep design review.

Before and After

before: spec.get("items")
after: spec.get("items", [])

Testing

Please describe the tests that you ran to verify your changes and how we can reproduce.

Checklist:

• I have performed manual end-to-end testing of the feature in my environment.
• I have added Docstrings and comments to the code.
• I have made changes to existing documentation where needed.
• I have added tests that show that the PR is functional.
• New and existing unit tests pass locally with my changes.
• I have added relevant collaborators to review the PR before merge.
• [Polymetis only] I ran on hardware (1) all scripts in tests/scripts, (2) asv benchmarks.

[codecov-commenter]

Codecov Report

Merging #1256 (3bdf77e) into main (5c0ff21) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1256      +/-   ##
==========================================
- Coverage   34.00%   33.92%   -0.09%     
==========================================
  Files         352      352              
  Lines       34848    34848              
==========================================
- Hits        11851    11823      -28     
- Misses      22997    23025      +28     

Impacted Files	Coverage Δ
agents/_agent.py	40.92% <0.00%> (-10.82%)	⬇️
lowlevel/minecraft/pyworld/fake_mobs.py	88.13% <0.00%> (ø)

[aszlam]

I am going to be rewriting a lot of this today, but this is fine for now to merge if its blocking

[jacklanchantin]

I am going to be rewriting a lot of this today, but this is fine for now to merge if its blocking

It's not blocking since I'm not generating data right now. So I'll leave it un-merged.

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