Don't assume that there will be an extra `context` kwarg when using the
bookmark serializer. We use it this way in the current code but that's
specific to us and not comon to all serializers. There are a lot of API
documentation tools that automate introspecting serializers but they
won't know that they have to send in a `context` to the serializer.
To make this serializer behave more like other serializers without
changing the behavior, we just need to check that the `context` value is
defined before we dig into it. In the case that there is no `context`
we just treat it the same as if there is no `request` in the `context`.
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