Issue #1554 fix XmlGenericMapAdapter to work with null maps.

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Former-commit-id: 7220c4c6cc
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edexOsgi/com.raytheon.uf.common.serialization/src/com/raytheon/uf/common/serialization/XmlGenericMapAdapter.java

@@ -56,6 +56,10 @@ public class XmlGenericMapAdapter<K, V> extends
 
     @Override
     public MapType<K, V> marshal(Map<K, V> unserialized) throws Exception {
+        if (unserialized == null) {
+            return null;
+        }
+
         MapType<K, V> mapType = new MapType<K, V>();
 
         for (Map.Entry<K, V> entry : unserialized.entrySet()) {