pallet-migrations: fix index access for singluar migrations

[ggwpez]

Discovered a bug in the migrations pallet while debugging https://github.com/paritytech/try-runtime-cli/pull/90.
🚨 This can lead to your chain getting stuck. It happens when a single MBM is configured. Having no or more than one MBM will circumvent the issue.

Changes:

• Check len of the tuple before accessing its nth_id
• Make nth_id return None on unary tuples and n>0

[ggwpez]

/cmd prdoc --pr 5695 --bump patch --audience "Runtime Dev"

[paritytech-cmd-bot-polkadot-sdk[bot]]

Git push to origin failed for stable2407 with exitcode 1

[paritytech-cmd-bot-polkadot-sdk[bot]]

Successfully created backport PR for stable2409:

• 5728

[paritytech-cmd-bot-polkadot-sdk[bot]]

Successfully created backport PR for stable2407:

• 5729

[paritytech-cmd-bot-polkadot-sdk[bot]]

Git push to origin failed for stable2409 with exitcode 1