fix: reject zero numerator in toXdrPrice fraction form

[rubenmarcus]

Fixes #1750.

toXdrPrice({ n: 0, d: 1 }) builds the Price without complaint: the fraction form only checks xdrObject.n < 0, so a zero numerator passes while every other way of expressing a zero price (numeric 0, string "0") throws price must be positive. The operation only fails later, when Stellar Core rejects it at submission, far from the invalid input.

The fix aligns the numerator check with the denominator check on the next line: n <= 0 || d <= 0.

Verification:

$ npx vitest run test/unit/base/operation.test.ts --config config/vitest.config.ts -t 'zero numeric price'
# on main: x throws 'price must be positive' for zero numeric price
#          AssertionError: expected [Function] to throw an error   <- toXdrPrice({ n: 0, d: 1 })

$ npx vitest run test/unit --config config/vitest.config.ts
Tests  6898 passed | 4 skipped | 9 todo (6911)

$ pnpm run typecheck:tests && pnpm run test:types
# both clean

Prepared with AI assistance (GLM 5.3 via Oh My Pi) and reviewed before submission.