REACH-82 "Null value cannot be cast to double"

When Number node is created it has `Value` is `null`. And only rendering causes updating it to `"0"`. Or changing `Value` from "0" to another number. That's why in headless mode when we created node number without changing its value (we need exactly "0") and connect it with another node, running is failed because that other node needs number rather than `null`
Nice work, @marimano! We have headless Dynamo just in time for Halloween! :)

Anyway, you could do this on the ```master``` branch if you wish, that way future changes in these lines on ```master``` will not cause too much merge headache. Having a ```headless``` option will not break ```master``` code, so it should be alright. I'm leaving it to you, if you need please let me know. Thanks again!