little fix for an error

[magelon]

The types returned by 'data(...)' are incompatible between these types. Type 'T' is not assignable to type 'DocumentData | undefined'. Type 'T' is not assignable to type 'DocumentData'.

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

Code sample

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