Check whether docsearch is defined before calling it (#358)

Prevents the docs from remaining hidden because of an error in theme.js as the result of e.g. the browser blocking the load of the docsearch script

Author: Simran-B

site/themes/arangodb-docs-theme/layouts/partials/search.html

@@ -4,6 +4,7 @@
 
 <script type="text/javascript">
   window.setupDocSearch = function (facetVersion) {
+    if (!window.docsearch) return;
     docsearch({
       appId: "OK3ZBQ5982",
       apiKey: "500c85ccecb335d507fe4449aed12e1d",