[faucet] Help typescript to infer the type of an empty array

Summary: In the config file the originWhitelist array is documented as possibly empty. This means that typescript has to infer the type from [] which causes a compilation failure.

Test Plan: Empty the array and run npm run build.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16877