Don't 'replace' isNone value with isNone value

In an environment with two sources connected with bi-directional transform-connectors, if you replace a value with a 'null', it creates an infinite loop. See this for an example: http://jsfiddle.net/chrisyunker/0neso67b/. It also does the same if you replace value with 'undefined'.

Adding another check for this situation in 'transform-connector.transform' is one way to fix this. However, there may be a better way.