Fix "Using null as the key parameter for array_key_exists" PHP 8.5 warning

Our implementation diverged from upstream Less.js 3.13.1 by using an
associative array instead of a list array. Change this so that null
values naturally work, without creating an amguity between null and
the empty string (since only strings can be valid array keys).

This change triggered a Phan warning, because Phan creates a simplified
analysis of the parseMixinArgs()['args'] array that mixes up the types
of the 'name' and 'value' fields, because it ignored the keys.

```
lib/Less/Tree/Mixin/Definition.php:45 PhanTypeMismatchProperty
Assigning ($p['name'] as a field) of type non-empty-array<int,?\Less_Tree|?string>
to property but \Less_Tree_Mixin_Definition->optionalParameters is string[]
```

Fix this by declaring the array shape.

Bug: T410596
Change-Id: Ic0b5274febea3d2318282619b5f762d11d2530f9

Author: del72683

lib/Less/Parser.php

@@ -1853,6 +1853,7 @@ private function parseMixinCallElements() {
 
 	/**
 	 * @param bool $isCall
+	 * @return array{args:array<array{name?:string,value?:mixed,variadic?:bool}>,variadic:bool}
 	 * @see less-2.5.3.js#parsers.mixin.args
 	 */
 	private function parseMixinArgs( $isCall ) {

lib/Less/Tree/Mixin/Definition.php

@@ -23,7 +23,7 @@ class Less_Tree_Mixin_Definition extends Less_Tree_Ruleset {
 	public $condition;
 	/** @var bool */
 	public $variadic;
-	/** @var array<string,true> */
+	/** @var string[] */
 	public $optionalParameters = [];
 
 	public function __construct( $name, $params, $rules, $condition, $variadic = false, $frames = [] ) {
@@ -38,10 +38,11 @@ public function __construct( $name, $params, $rules, $condition, $variadic = fal
 		if ( $params ) {
 			$this->arity = count( $params );
 			foreach ( $params as $p ) {
-				if ( !isset( $p['name'] ) || ( $p['name'] && !isset( $p['value'] ) ) ) {
+				// NOTE: Less.js 3.13.1 does a !p.name check in the second half that we omit, because it is impossible.
+				if ( !isset( $p['name'] ) || !isset( $p['value'] ) ) {
 					$this->required++;
 				} else {
-					$this->optionalParameters[ (string)$p['name'] ] = true;
+					$this->optionalParameters[] = $p['name'];
 				}
 			}
 		}
@@ -239,7 +240,9 @@ public function matchArgs( $args, $env = null ) {
 		$allArgsCnt = count( $args );
 		$requiredArgsCnt = 0;
 		foreach ( $args as $arg ) {
-			if ( !array_key_exists( $arg['name'], $this->optionalParameters ) ) {
+			// NOTE: A positional mixin arg will have a name of null in Less_Tree_Mixin_Call::compile,
+			// which is never in the optionalParameters array.
+			if ( !in_array( $arg['name'], $this->optionalParameters, true ) ) {
 				$requiredArgsCnt++;
 			}
 		}