fix: prevent error in hardhat-viem when using a chain not included in viem's predefined list

[stephen-lumi]

Fixed #6854

• [x] Because this PR includes a bug fix, relevant tests have been included.
• [ ] Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• [ ] I didn't do anything of this.

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[galargh]

Hey! I've just commented on the related issue - https://github.com/NomicFoundation/hardhat/issues/6854#issuecomment-2976611908

If it resolves the issue for you, we should be able to close this PR.

[galargh]

I'll close this as per https://github.com/NomicFoundation/hardhat/issues/6854#issuecomment-3045573521 but thank you for the PR!