[mv3] Fix export button in "Custom filters" pane

Author: gorhill

platform/mv3/extension/js/filter-manager-ui.js

@@ -532,7 +532,8 @@ function importFromFile() {
 function exportToFile() {
     const lines = [];
     for ( const hostnameNode of qsa$('.hostnames li.hostname') ) {
-        const hostname = punycode.toUnicode(hostnameFromNode(hostnameNode));
+        const hostname = punycode.toUnicode(hostnameFromNode(hostnameNode) || '');
+        if ( hostname === '' ) { continue; }
         const selectors = selectorsFromNode(hostnameNode);
         for ( const selector of selectors ) {
             lines.push(`${hostname}##${toPrettySelector(selector)}`);