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
In https://github.com/babel/babel/issues/9511 (and #9495 is another symptom), @PavelKastornyy reported a node crash becaue the JavaScript heap run out of memory. The problem was that their code was adding enumerable properties to `Object.prototype`: it is something that shouldn't be done, but Babel shouldn't make node crash if someone adds them.
I reduced down the problem to `for...in` loops in `@babel/traverse` that grew the memory consumption exponentially because of that unexpected properties.
