[libclang/python] Return None instead of null cursors from Token.cursor (#163183)

Since https://github.com/llvm/llvm-project/pull/138103 , the `Cursor`
class throws an error when any of its methods is called on a null
cursor. Simultaneously, we adapted all methods to return `None` instead
of a null cursor, so users should not encounter these. We have
overlooked one way to end up with null cursors, namely the
`Token.cursor` property, which may return null cursors under some
circumstances.

Fixes #163180

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