fix panic when setting empty iap block

[danawillow]

Fixes #1231

[paddycarver]

Ideally this would come with a test that exercised the panic, but I'm not going to block it on that.

[danawillow]

I don't want to set a precedent for that- already this is a super weird case, and if we added tests for every potential empty block we would end up with a huge number of tests.

However, if you really wanted one I would happily review the PR :)

[paddycarver]

I'm in alignment with you on that one :)

[ghost]

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