mergeDeep behavior changed in 10.10.0 for null values

[pbomb]

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Describe the bug

In 10.9.1 and earlier, passing in a object with a property whose value is null no longer overrides the property value from previous objects.

To Reproduce Steps to reproduce the behavior:

mergeDeep([{ a: foo }, { a: null }])
This results in { a: foo } now

Expected behavior

It used (and is expected) to result in { a: null }

Environment:

• OS: MacOS
• @graphql-tools/utils: 10.10.0+
• NodeJS: 24.14.0

Additional context