fix(migrations): fix broken migration when no control flow is present

[jessicajaniuk]

This addresses a bug that caused the control flow migration to crash when no control flow was present in the template.

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix

Does this PR introduce a breaking change?

• Yes
• No

[jessicajaniuk]

caretaker: failing test is unrelated. this is safe to merge.

[dylhunn]

This PR was merged into the repository by commit f1a020b.

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