refs #18290 [finc] add caching for fallback covers

* also removed redundant array-check in Router

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

@@ -41,6 +41,8 @@ namespace finc\Cover;
  */
 class Loader extends \VuFind\Cover\Loader
 {
+    const EMPTY_IMAGE_CONTENT = 'nothing here';
+
     /**
      * Flag denoting the last loaded image was a FailImage
      *
@@ -57,9 +59,26 @@ class Loader extends \VuFind\Cover\Loader
     public function loadUnavailable()
     {
         $this->hasLoadedUnavailable = true;
+        file_put_contents($this->localFile,self::EMPTY_IMAGE_CONTENT);
         return parent::loadUnavailable();
     }
 
+    public function loadImage($settings = [])
+    {
+        parent::loadImage($settings);
+        if ($this->image === self::EMPTY_IMAGE_CONTENT) {
+            $this->hasLoadedUnavailable = true;
+        }
+    }
+
+    public function getImage()
+    {
+        if ($this->image === self::EMPTY_IMAGE_CONTENT) {
+            $this->hasLoadedUnavailable = true;
+        }
+        return parent::getImage();
+    }
+
     /**
      * Returns true if the last loaded image was the FailImage
      *

module/finc/src/finc/Cover/Router.php

@@ -98,10 +98,8 @@ class Router implements \Zend\Log\LoggerAwareInterface
             if (!$resolveDynamic) {
                 return null;
             }
-            $settings = is_array($thumb) ? array_merge($thumb, ['size' => $size])
-                : ['size' => $size];
             if ($testLoadImage) {
-                $this->coverLoader->loadImage($settings);
+                $this->coverLoader->loadImage(array_merge($thumb, ['size' => $size]));
                 if ($this->coverLoader->hasLoadedUnavailable()) {
                     return false;
                 }