[Php74] Add IfToNullCoalescingAssignRector to convert if null-guard to ??=

[TomasVotruba]

Closes rectorphp/rector#9843

Adds IfToNullCoalescingAssignRector (Php74 set). It converts an if null-guard around a single self-assignment into the ??= null coalescing assignment operator.

Three guard shapes are recognized:

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

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

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

Safety guards (rule bails out otherwise)

• No else / elseif branch.
• Exactly one statement in the if body, and it is a plain assignment.
• The assigned target matches the tested expression.
• The assigned value does not reference the target (skips $x = $x . 'suffix').
• isset() with more than one variable is skipped.

Both operand orders of the identical check (null === $x and $x === null) are handled.

The rule dogfoods on rector-src itself (8 files simplified in this PR).