Fix issue with richtext color values not being read in Xlsx.php (prob…

…ably related to a namespace change and - according to commit log - issue PHPOffice#2303)

Use SimpleXMLElement::count() instead of the native count function to avoid code style issues.

src/PhpSpreadsheet/Reader/Xlsx.php

@@ -1689,7 +1689,7 @@ private function parseRichText(?SimpleXMLElement $is)
                             $objText->getFont()->setSize((float) $attr['val']);
                         }
                         $attr = $run->rPr->color->attributes();
-                        if (count($attr) > 0) {
+                        if ($attr && $attr->count() > 0) {
                             $objText->getFont()->setColor(new Color($this->styleReader->readColor($attr)));
                         }
                         if (isset($run->rPr->b)) {