fix: Update transaction data property to be nullable

[usame-algan]

What it solves

Part of safe-global/safe-wallet-monorepo#1890

In #117, we updated the type for data in DecodedDataResponse to be nullable but the corresponding type DataDecoded still had data to be optional which causes a type mismatch.

How it solves it

• Update data in DataDecoded to be string | null

[github-actions]

ESLint Summary View Full Report

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[katspaugh]

Thanks! Please bump the version again, I'll publish it