fix: validate amount before calling parseUnits in _getPriceParameters

[Sertug17]

Problem

In _getPriceParameters the null guard was placed after amount.toString() was already called:

// BEFORE (buggy)
const amountWei = parseUnits(amount.toString(), decimals) // crashes if null
if (amount == null || amountWei < 0) { throw ... }        // too late

If amount is null or undefined (possible from JavaScript callers or edge cases), this produces an unhandled TypeError: Cannot read properties of null (reading 'toString') instead of the intended validation error.

Fix

Move the null check — and the isEther/OFFER precondition — before parseUnits is called. The negative-bigint guard stays after since it requires amountWei to exist:

// AFTER (fixed)
if (amount == null) { throw new Error('Starting price must be a number >= 0') }
if (isEther && orderSide === OrderSide.OFFER) { throw ... }
const amountWei = parseUnits(amount.toString(), decimals) // safe now
if (amountWei < 0) { throw new Error('Starting price must be a number >= 0') }

Testing

Added a regression test in test/sdk/misc.spec.ts confirming that passing null as amount does not surface a raw TypeError.