fix(Hex): throw on empty hex in toNumber instead of returning NaN

[spokodev]

Hex.toNumber('0x') returns NaN, while Hex.toBigInt('0x') throws.

The no-options fast path decodes via Number(hex), and Number('0x') is NaN, so empty hex silently yields a non-number instead of erroring. viem's hexToNumber routes through hexToBigInt and throws here; Bytes.toNumber (which delegates to Hex.toNumber) is affected too.

The fix keeps the fast path for valid hex and only defers to toBigInt when Number(hex) is NaN. For all valid hex, Number(hex) === Number(toBigInt(hex)); 0x is the only divergence, so no valid result changes.

Added a regression test asserting Hex.toNumber('0x') throws.

[vercel]

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