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ARROW-12799: [JS] Use Nullish Coalescing Operator (??) For Defaults

**The nullish coalescing operator (`??`) returns its right side when its left side is nullish** (`null` or `undefined`), and its left side otherwise.
For example, `const x = a ?? b` would set `x` to `a` if `a` has a value, and to `b` if `a` is `null` or `undefined`.

The nullish coalescing operator is very useful to **provide default values when a value or an expression is nullish**.
Before its introduction in ES2020, this default value pattern was often expressed using the conditional operator.

This refactoring simplifies conditional (ternary) checks to nullish coalescing operator expressions:

* `a == null ? x : a` becomes `a ?? x`
* `a != null ? a : x` becomes `a ?? x`
* `a === null || a === undefined ? x : a` becomes `a ?? x`
* `a !== null && a !== undefined ? a : x` becomes `a ?? x`
* `f(1) != null ? f(1) : x` becomes `f(1) ?? x`
* etc.

Learn More: [Nullish coalescing operator (MDN)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator)

When two similar-looking function calls have a side effect, this refactoring can change the behavior of the code.

For example, the refactoring changes:

```javascript
let a = f(1) === null || f(1) === undefined ? 'default' : f(1);
```

into

```javascript
let a = f(1) ?? 'default';
```

If `f(1)` has a side effect, it would have been called one, two or three times before the refactoring, and once after the refactoring.
This means that the side effect would have been called a different number of times, potentially changing the behavior.

Closes #10339 from domoritz/p42/nullish_coalescing_operator/1621098556103

Authored-by: P42 <72252241+p42-ai[bot]@users.noreply.github.com>
Signed-off-by: Dominik Moritz <domoritz@gmail.com>