[CodeQuality] Skip SimpleXMLElement on IssetOnPropertyObjectToPropertyExistsRector

[TomasVotruba]

SimpleXMLElement resolves properties from the XML document at runtime, not from the class. property_exists() always returns false for them, so the rewrite breaks working code.

 public function x(SimpleXMLElement $svg) {
     $svgAttributes = $svg->attributes();

-    if (isset($svgAttributes->width) && isset($svgAttributes->height)) {
+    if (property_exists($svgAttributes, 'width') && $svgAttributes->width !== null && (property_exists($svgAttributes, 'height') && $svgAttributes->height !== null)) {
         return true;
     }

-    return isset($svg->viewBox);
+    return property_exists($svg, 'viewBox') && $svg->viewBox !== null;
 }

Now SimpleXMLElement and its children are skipped.