[ecash-herald] Patch error in parsing burn txs

Summary:
T2972

Ran into this error running some tests. This tx threw an error because it had tokenBurnInfo but no associated tokenId.

Discussed with Tobias and I've made some incorrect assumptions about how chronik indexes token burns. Will correct this in a later diff. This just patches a potential edge case and makes the app more robust to errors.

Test Plan:
npm test
npm run sendMsgByBlock 798201 and no error

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14158