[Php74] Add rule to convert `if (!isset($x)) { $x = ...; }` to `??=`

[Soean]

Feature Request

NullCoalescingOperatorRector currently operates on Assign nodes, so it only recognizes the self-referencing coalesce form. The equivalent if guard which is what most pre-7.4 code actually looks like, is not detected.

Note that Rector already encodes this equivalence in the opposite direction: the PHP 7.4 downgrade set rewrites ??= into exactly the if ( ! isset( ... ) ) block requested here (see #4124). Only the upgrade direction is missing.

Diff

1. isset() guard

-if (!isset($array['user_id'])) {
-    $array['user_id'] = 'value';
-}
+$array['user_id'] ??= 'value';

2. null === guard

-if (null === $value) {
-    $value = 'default';
-}
+$value ??= 'default';

3. is_null() guard

-if (is_null($this->instance)) {
-    $this->instance = new self();
-}
+$this->instance ??= new self();

Conditions

The transformation is only safe when all of these hold, and skipping any of them changes behavior:

• The if has no else or elseif branch.
• The body contains exactly one statement. Anything else in the block would move from running once to running on every pass:

  // must NOT be converted
  if (null === self::$instance) {
      self::$instance = new self();
      do_action('init', self::$instance); // would fire on every call
  }

• The condition is not compound. if (null === $args['a'] || !$args['b']) is not equivalent.
• The assignment target is the same expression the condition tests.
• The assigned value does not reference the target itself ($x = $x . 'a' would be evaluated differently).