Allow null in url strings

[robinweser]

Description

This PR adds a special check for url() values since null and undefined could potentially be part of a Base64 string as seen in #772.

Packages

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• fela-utils

Versioning

Patch

Checklist

Quality Assurance

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• The code was formatted using Prettier (yarn run format)
• The code has no linting errors (yarn run lint)
• All tests are passing (yarn run test)
• There are no flow-type errors (yarn run flow)

Changes

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• Tests have been added/updated
• Documentation has been added/updated
• My changes have proper flow-types