Remove checks for None for fields which should never be None

[hmellor]

These fields of VllmConfig should never None and so allowing them to be None in VllmConfig.__post_init__ could lead to errors in consumers of VllmConfig which (rightly) assume that the field should never be None

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @hmellor.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork