[explorer] Fix unconfirmed txs showing as block height 0

Summary: Block height is None for mempool transactions, so update the code accordingly.

Test Plan: Watch your wallet address and send yourself a transaction. Check it shows as unconfirmed with a red background and is non-clickable, instead of height 0 with an invalid link.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16306