Extend mapping for alternative titles

src/MDConcTitleTypes.php

@@ -16,6 +16,9 @@ final class MDConcTitleTypes implements MDImporterConcordanceListInterface {
 
         // Empty
         "" => "",
+        "Objekttitel" => "",
+        "Werktitel" => "",
+        "Werktitel deskriptiv" => "",
 
         // Scientific names
         "Science" => "Science",