Add default values in Border dataclass

[zrr1999]

When I customize the sidebar, I want there to be a dividing line on the right, so I use the Container class and add the border parameter. In this process, I found that the Border class must pass in all four parameters, but I want the effect to be

Border(right=BorderSide(2, colors.PRIMARY))

now it must be

Border(None, BorderSide(2, colors.PRIMARY), None, None)

The pr add default values in Border dataclass.

[CLAassistant]

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