pallet-migrations: fix index access for singluar migrations (#5695)

Discovered a bug in the migrations pallet while debugging
https://github.com/paritytech/try-runtime-cli/pull/90.
It only occurs when a single MBM is configured - hence it did not happen
when Ajuna Network tried it...

Changes:
- Check len of the tuple before accessing its nth_id
- Make nth_id return `None` on unary tuples and n>0

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: ggwpez <[email protected]>
(cherry picked from commit 01364633)