[Cashtab] Handle parsing errors for off-spec alias registrations

Summary: parseChronikTx needs a fallback if parsedOpReturnArray[2] does not exist, as may be the case for pre-spec test alias registrations or any tx with the alias registration lokad id in the OP_RETURN.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15274