fix background-image: none

[cathor01]

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Summary

This PR fixes/implements the following bugs/features
throw unsupported image when using background-image: none

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Demonstrate how the issue/feature can be replicated. For most cases, simply adding an appropriate html/css template into the reftests should be sufficient. Please see other tests there for reference.

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[niklasvh]

What issue does this fix? Can you provide a test case where the issue can be reproduced?