code: public static boolean isValidPropertyNotNull(final GraphObject node, final PropertyKey key, final ErrorBuffer errorBuffer) {
  
  		final String type  = node.getType();
  		if (key == null) {
  
  			errorBuffer.add(new EmptyPropertyToken(type, UnknownType));
  			return false;
  		}
  
  		final Object value = node.getProperty(key);
  		if (value != null) {
  
  			if (value instanceof Iterable) {
  
  				if (((Iterable) value).iterator().hasNext()) {
  
  					return true;
  				}
  
  			} else {
  
  				return true;
  
  			}
  		}
  
  		errorBuffer.add(new EmptyPropertyToken(type, key));
  
  		return false;
  	}
docs: Checks whether the value for the given property key of the given node
  is a non-empty string.
  
  @param node the node
  @param key the property key
  @param errorBuffer the error buffer
  
  @return true if the condition is valid