[native] Don't crash if Alchemy API key is missing

Summary: This fixes ENG-2836. I should've tested this scenario before landing D6373. The docs seemed to indicate that the Alchemy API key was optional, and I know Wagmi "works" without it. But now I think it's only optional for Ethers.js for testnet or something?

Test Plan: Confirm crash no longer occurs when Alchemy API key is missing (no native/facts/alchemy.json file) in native app

Reviewers: kamil, przemek, atul, ginsu, marcin, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D6396