This fixes an issue where destructuring assignments eligible for the "array unpacking" optimization would fail to compile when the array literal on the right-hand side of the expression contained holes.
Example input:
```js
[a, b] = [, 2];
; // Avoid completion record special case
```
The error message was `Property right of AssignmentExpression expected node to be of a type ["Expression"] but instead got null`.
Now the above code compiles to:
```js
a = void 0;
b = 2;
;
```
This PR also adds a couple of related test cases that were missing, to ensure the change doesn't regress them:
* Normal assignment expression with unpacking
* Declaration with unpacking and a hole on RHS
* No unneeded empty arrays in transform spread
Since Array.prototype.concat creates a new array from inputs, there's
no need to call it from a new empty array ([].concat()).
* [fixup] simplify detection of new array
