[Cashtab] Fallback to block time if timeFirstSeen is zero

Summary:
T2806

If chronik has not seen a tx, timeFirstSeen will be zero and Cashtab will parse the tx as having been sent in 1970.

Fall back to blocktime in this case to get a mostly accurate time estimate.

Test Plan:
Ping me on Telegram for a wallet seed with some txs in this category
Import at cashtab.com and note wrong dates
`npm start` and confirm accurate dates

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12648

Author: BytesOfMan

web/cashtab/src/components/Home/Tx.js

@@ -437,7 +437,10 @@ const Tx = ({
     const handleShowMessage = () => {
         setDisplayedMessage(!displayedMessage);
     };
-    const txDate = formatDate(data.timeFirstSeen, navigator.language);
+    const txDate =
+        data.timeFirstSeen !== '0'
+            ? formatDate(data.timeFirstSeen, navigator.language)
+            : formatDate(data.block.timestamp, navigator.language);
 
     // A wallet migrating from bch-api tx history to chronik will get caught here for one update cycle
     let unparsedTx = false;