Optimize border-radius flip by using native 'strlen' callback

Before:
* input.css  iterations=10000 max=2.99ms mean=0.09ms rate=10627/s
* input.css  iterations=10000 max=2.88ms mean=0.09ms rate=10565/s

After:
* input.css  iterations=10000 max=2.89ms mean=0.07ms rate=13890/s
* input.css  iterations=10000 max=3.45ms mean=0.07ms rate=13915/s

Based on the bench.php option from If86b1806e6f, using PHP 8.1, and input of:

```
.foo {
  border-radius: 2px 1px 3px;
  border-radius: 2px 0 0 2px;
  border-radius: 0 2px 2px 0;
  border-radius: 0;
  border-radius: 50% 1em;
}
```

Change-Id: I712be1c4b3dd720d250ba32ba2534fdf8874fda1

Author: thiemowmde

src/CSSJanus.php

@@ -322,12 +322,8 @@ private static function fixBorderRadius($css) {
 	 */
 	private static function calculateBorderRadius($matches) {
 		$pre = $matches[1];
-		$firstGroup = array_filter(array_slice($matches, 2, 4), function ($match) {
-			return $match !== '';
-		});
-		$secondGroup = array_filter(array_slice($matches, 6, 4), function ($match) {
-			return $match !== '';
-		});
+		$firstGroup = array_filter(array_slice($matches, 2, 4), 'strlen');
+		$secondGroup = array_filter(array_slice($matches, 6, 4), 'strlen');
 		$post = $matches[10] ?: '';
 
 		if ($secondGroup) {