Harden MarcXmlEncoder

Author: dr0i

metafacture-biblio/src/main/java/org/metafacture/biblio/marc21/MarcXmlEncoder.java

@@ -175,7 +175,8 @@ public void literal(final String name, final String value) {
         if (currentEntity.equals("")) {
             prettyPrintIndentation();
             writeRaw(String.format(CONTROLFIELD_OPEN_TEMPLATE, name));
-            writeEscaped(value.trim());
+            if (value!=null)
+                writeEscaped(value.trim());
             writeRaw(CONTROLFIELD_CLOSE);
             prettyPrintNewLine();
         } else if (!currentEntity.equals(Marc21EventNames.LEADER_ENTITY)) {