refs #18741 [finc] fix unwanted caching of covers if cover fallbacks are disabled

module/finc/src/finc/Cover/Loader.php

@@ -59,7 +59,9 @@ class Loader extends \VuFind\Cover\Loader
     public function loadUnavailable()
     {
         $this->hasLoadedUnavailable = true;
-        file_put_contents($this->localFile,self::EMPTY_IMAGE_CONTENT);
+        if ($this->config->Content->useCoverFallbacksOnFail) {
+            file_put_contents($this->localFile, self::EMPTY_IMAGE_CONTENT);
+        }
         return parent::loadUnavailable();
     }