Change AEC last() to assert agent is not None

[elliottower]

Previously this would fail if AgentID was zero, which is incorrect behavior

Fixes #1119

Description

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Fixes # (issue), Depends on # (pull request)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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Checklist:

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• I have run pytest -v and no errors are present.
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