Fixes #2834. v1 Border BorderBrush and Background wrongly sets to an invalid Color enum.

[BDisp]

Fixes #2834 - The borderBrush and background private fields are a nullable Color type to specify if they are defined but to avoid breaking changes the BorderBrush and Background couldn't been also set to a nullable Color type, because it would force the users to do Color casting. So, the solution is to avoid them be set to a non defined Color.

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