Issue #2471737 by klausi: PageCache::get() should return FALSE on a cache miss...

Issue #2471737 by klausi: PageCache::get() should return FALSE on a cache miss as promised by the docs

core/modules/page_cache/src/StackMiddleware/PageCache.php

@@ -255,6 +255,7 @@ protected function get(Request $request, $allow_invalid = FALSE) {
     if ($cache = $this->cache->get($cid, $allow_invalid)) {
       return $cache->data;
     }
+    return FALSE;
   }
 
   /**