Fix minor bug in TerminateIllegal wrapper indexing empty info dict

[elliottower]

This fixes an issue with this PR Farama-Foundation/SuperSuit#233

The tests all now pass with that PR locally, was just due to indexing an empty infos dict which does not contain the agent ID. This happens at termination, when you try to observe, there are no agents therefore there is nothing in the info dict or terminations dicts either, so it doesn't make sense to check infos[self.agent_selection] in that case.

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