Added a fallback value for useragent (#20172)

Without this change, some browsers, like GoogleBot, don't properly render the page.
Instead of weird JS exception (and hit to server with 404) we should at least try
to render the app with all browser. As webikit/chromium based browsers are
nowadays most popular, falling back to "safari" permutation.

Change-Id: I4275110962432b2552d51715a86a8e815fe539ad

client/src/main/resources/com/vaadin/Vaadin.gwt.xml

@@ -60,6 +60,9 @@
             <when-property-is name="user.agent" value="gecko1_8" />
         </none>
     </set-property>
+    
+    <!-- If no proper user agent is found, at least try some, e.g. with crawlers -->
+    <set-property-fallback name="user.agent" value="safari" />
 
     <!-- Fall through to this rule when the browser doesn't support pointer 
         event -->