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`
It seems like the best way is to do this when the data member is declared (imagine we add another constructor to ```DoubleInput``` class and that forgets to set this):

```cs
private string _value = ```0```;
```