fix: use null instead of 0 for testnet token USD fallback

When price is null (testnet), the fallback value 0 caused formatUsd()
to display "$0.00" instead of hiding the USD value. Using null makes
formatUsd() return an empty string, so the UI correctly shows no USD
on the token detail view.

src/popup/views/addressToken.js

@@ -161,7 +161,7 @@ function show() {
         `<a href="${addrLink}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
 
     // USD total for this token only
-    const usdVal = price ? amount * price : 0;
+    const usdVal = price ? amount * price : null;
     const usdStr = formatUsd(usdVal);
     $("address-token-usd-total").innerHTML = usdStr || "&nbsp;";