[base] Fix wrong typeof check

[bjoerge]

Currently, when restoring a document from the revision history we look through the restored version for references to check if they still exist. Due to an erroneous typeof check, which should have been a falsey check, any null values in the restored version would throw an error saying Cannot read property _ref of null.

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Note for release
Fixed a bug that would cause an error when trying to restore a document that has null values.

[vercel]

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[thomax]

👍