Fixes a core bug which causes full cache clean to happen during mview reindex

$cacheContext->getIdentities() will return an empty array due to the "DeferredCacheContext" class introduced in 2.4.5.
If an empty array is passed to clean(), it will trigger a full cache clean.

https://www.wrike.com/open.htm?id=1018895699


--- Model/FlushCacheByCacheTag.php
+++ Model/FlushCacheByCacheTag.php
@@ -59,7 +59,9 @@ class FlushCacheByCacheTag
             $cacheContext = $this->cacheContextFactory->create();
             $cacheContext->registerEntities($cacheTag, $entityIds);
             $this->eventManager->dispatch('clean_cache_by_tags', ['object' => $cacheContext]);
+            if ($cacheContext->getIdentities()) {
             $this->appCache->clean($cacheContext->getIdentities());
+            }
         }
     }
 }