Preserve None for reset event terms

[kgu42]

Description

Fixes #7791.

EventManager.apply(mode="reset", env_ids=None) converted None to
slice(None) for internal reset bookkeeping. Unfiltered event terms then
received that slice instead of None, causing built-in terms that expect
torch.Tensor | None to fail.

This change keeps the caller-provided env_ids unchanged and uses a separate
resolved_env_ids selector for internal bookkeeping.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Release backport

• Backport this pull request to the active release branch after it merges into develop

Screenshots

N/A

Testing

• ./isaaclab.sh -p -m pytest source/isaaclab/test/managers/test_event_manager_unit.py -q
• ./isaaclab.sh -f

Checklist

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• I have read and understood the contribution guidelines
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• I have added my name to the CONTRIBUTORS.md or my name already exists there

[kgu42]

run-ci