How to build Merkle root from merkle branches?
This would indicate an empty block (no transactions). So only coinbase should be used to construct merkle root. So merkle root = Coinbase

Example block: https://www.blockchain.com/btc/block-index/512049/00000000000000000429a0c4fbe735b2d8b493daedf0207728543f748c262437

(edit: but please verify with others)