[Php74] Keep comments above if on IfToNullCoalescingAssignRector

[Soean]

IfToNullCoalescingAssignRector builds a fresh Expression node for the ??= statement and returns it. A new node carries no attributes, so the COMMENTS of the replaced If_ are dropped — every docblock or line comment above the guard is lost.

Before (rule fires, comment disappears)

/**
 * Fallback value description
 *
 * @var int $limit
 */
if (! isset($limit)) {
    $limit = 100;
}

// simple comment
if (! isset($name)) {
    $name = 'default';
}

...becomes this — both comments are gone

$limit ??= 100;

$name ??= 'default';

Fix

Mirror the comments from the If_ onto the new Expression before returning it.

After

/**
 * Fallback value description
 *
 * @var int $limit
 */
$limit ??= 100;

// simple comment
$name ??= 'default';

All three guard forms (! isset(), is_null(), === null) share the single return path, so one fixture covers them.

[TomasVotruba]

Looks good, thank you