Improve error handling of `generateSeedEthereum` in design system shared

[georgewrmarshall]

https://github.com/MetaMask/metamask-design-system/pull/817/files#r2294831748

Bug: Ethereum Seed Generation Fails With Invalid Inputs

The generateSeedEthereum function can return NaN because parseInt receives an empty string (for short addresses like '0x') or invalid hexadecimal characters from the sliced address. This may cause issues for downstream icon generation expecting a numeric seed.