Add Google Analytics tracking for searches that return no results.

[karenmajewicz]

Instructions from GA:

Track zero-result searches in GA4 via GTM dataLayer
Summary
Add a dataLayer push that fires when an internal search returns zero results, enabling GA4 to capture no-result search terms via Google Tag Manager.

Implementation
Push the following event when a search response returns zero results. Populate search_term dynamically from the active query string (search?q=):

dataLayer.push({
'event': 'search_no_results',
'search_term': '<query_string_value>'
});

Validation
Verify event appears in GTM Preview mode with correct event name and search_term value
Confirm push fires only on zero-result searches