Fixes MediaCacheRefresher, it wasn't checking if there was an IMedia cache

src/Umbraco.Web/Cache/MediaCacheRefresher.cs

@@ -47,15 +47,17 @@ namespace Umbraco.Web.Cache
 
             if (anythingChanged)
             {
-                var mediaCache = CacheHelper.IsolatedRuntimeCache.GetCache<IMedia>();
-
                 Current.ApplicationCache.ClearPartialViewCache();
 
+                var mediaCache = CacheHelper.IsolatedRuntimeCache.GetCache<IMedia>();
+
                 foreach (var payload in payloads)
                 {
                     if (payload.ChangeTypes == TreeChangeTypes.Remove)
                        _idkMap.ClearCache(payload.Id);
 
+                    if (!mediaCache) continue;
+
                     // repository cache
                     // it *was* done for each pathId but really that does not make sense
                     // only need to do it for the current media