title: JsonGenerationException: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)
body: <p>I'm trying to add an instance of this class to <code>neo4j spring 4.0.0.RELEASE</code>:</p>
  
  <pre><code>@NodeEntity
  public class User
  {
      @GraphId
      private Long id;
      @Property
      private UUID uniqueId = UUID.randomUUID();
      @Property
      private String username;
      @Property
      private String password;
  
      // getters, setters, blah blah...
  }
  </code></pre>
  
  <p>When I do I get this exception:</p>
  
  <pre><code>ERROR o.a.c.c.C.[.[.[.[dispatcherServlet] - Servlet.service() for servlet [dispatcherServlet] in context with path [/pegboard] threw exception [Request processing failed; nested exception is org.neo4j.ogm.metadata.MappingException: Could not create JSON due to Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)] with root cause
  com.fasterxml.jackson.core.JsonGenerationException: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)
      at com.fasterxml.jackson.databind.ser.impl.FailingSerializer.serialize(FailingSerializer.java:35) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:538) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:469) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:29) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:561) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:469) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:29) ~[jackson-databind-2.6.5.jar:2.6.5]
      at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:693) ~[jackson-databind-2.6.5.jar:2.6.5]
  ...more stack trace...
  </code></pre>
  
  <p>What is going on?</p>