fix(masthead-search): no redirect on empty value

This change ensures redirect to dedicated search page upon clicking on
the loupe icon happens only when there is a search query in the input
box.

Refs carbon-design-system#3883.

packages/web-components/src/components/masthead/masthead-search.ts

@@ -88,7 +88,9 @@ class DDSMastheadSearch extends BXDropdown {
   private async _handleClickSearchButton() {
     const { active } = this;
     if (active) {
-      this._handleUserInitiatedRedirect();
+      if (this._searchInputNode.value) {
+        this._handleUserInitiatedRedirect();
+      }
     } else {
       this.active = true;
       await this.updateComplete;