fixed speech overview for some invalid date strings

src/main/java/org/texttechnologylab/project/gruppe_05_1/database/MongoPprUtils.java

@@ -381,7 +381,11 @@ public class MongoPprUtils {
 
         // Sortiere nach Datum, absteigend
         speechMetaDataList.sort((md1, md2) -> {
-            return md2.getDateTime().compareTo(md1.getDateTime());
+            try {
+                return md2.getDateTime().compareTo(md1.getDateTime());
+            } catch (NullPointerException e) {
+                return 0;
+            }
         });
 
         return speechMetaDataList;