code: public static <A, B> void updateMapAndRemoveNulls(Map<A, B> source, Map<A, B> target) {
  
          assert source != target;
          for (Map.Entry<A, B> entry : source.entrySet()) {
              A key = entry.getKey();
              B value = entry.getValue();
              if (value != null) {
                  target.put(key, value);
              } else {
                  target.remove(key);
              }
          }
      }
docs: Copies entries from one map to another and deletes those entries in the target map for which
  the value in the source map is null.<p>
  
  @param <A> the key type of the map
  @param <B> the value type of the map
  @param source the source map
  @param target the target map