feat: add Word::empty() method to Miden SDK

[PivasDesant]

Fixes #822

I've added a Word::empty() method that returns a Word with all four field elements set to zero. This replaces the repetitive Word::from([Felt::from_u32(0); 4]) pattern we had scattered across the test files.

Updated all 5 occurrences in the integration tests to use the new method.

[bitwalker]

Thanks! For future reference, we'd prefer if unsolicited PRs refer back to a previously reported issue in the tracker, to avoid people working on things that we are already working on, or that might be wasted effort (e.g. updating code that is being removed). In general we close/reject PRs that don't follow that process.

I've left a comment here with a change I'd like to make before this is merged, other than that, looks good to me.

[bitwalker]

Let's implement this via the Default trait instead, which makes it more generally useful anyway.

[PivasDesant]

Thanks! For future reference, we'd prefer if unsolicited PRs refer back to a previously reported issue in the tracker, to avoid people working on things that we are already working on, or that might be wasted effort (e.g. updating code that is being removed). In general we close/reject PRs that don't follow that process.

I've left a comment here with a change I'd like to make before this is merged, other than that, looks good to me.

So before doing an issue I should be assigned to it?