Fix panic when planning orphaned deposed instances

[alisdair]

This is a highly local fix for a panic which seems to arise when planning orphaned deposed instances. There are two other things to investigate in response to this issue, likely addressing each in follow-up PRs:

• Fix the graph transformers so that we don't add orphan processing nodes when the instance is deposed;
• Investigate why readResourceInstanceState needs to return nil, nil in the first place, and redesign it if possible so that we don't have this class of bug elsewhere.

(I may have misunderstood an out-of-band discussion about this, and if we might not want to use this approach at all, in which case we'll probably just fix the graph transformers instead.)

Fixes #32645 (probably; we don't have a full reproduction to validate this)

Target Release

1.3.x

This is a panic and a narrowly-scoped fix, so I think we should backport it.

Draft CHANGELOG entry

BUG FIXES

• Fix crash when planning to remove already-deposed resource instances.

[github-actions]

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[github-actions]

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