Check if file exists before deleting in destroy()

Added file_exists check before deleting session file in destroy() of VuFind/Session/File.php to prevent Warning: unlink(): No such file or directory

module/VuFind/src/VuFind/Session/File.php

@@ -140,7 +140,10 @@ class File extends AbstractBase
 
         // Perform file-specific cleanup:
         $sess_file = $this->getPath() . '/sess_' . $sess_id;
-        return(unlink($sess_file));
+        if (file_exists($sess_file)) {
+            return(unlink($sess_file));
+        }
+        return true;
     }
 
     /**
@@ -160,4 +163,4 @@ class File extends AbstractBase
         }
         return true;
     }
-}
+}
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