[cpp] Cast from a null typed Array to Array<Dynamic> results in an empty array

We had a really odd game-breaking bug reported after shipping an update using 3.4.0 (and since repeated using 3.4.2 and HXCPP 3.4.64) which we tracked down to the behaviour demonstrated in the following example:
```
var test:Array = null;
var test2:Array = test;
trace(test2);
```
Resulting in test2 being an empty array rather than null (which the code relied on).
```
Main.hx:6: []
```
We worked around the issue and released updates, so it's not an emergency, but this doesn't seem right to me.