🐛 Fix strikethrough XML output for font styles

Word can't have strikethrough and double strikethrough declared and thus
ignores both.

PHPOffice#2661

Author: CWDN

src/PhpWord/Writer/Word2007/Style/Font.php

@@ -117,8 +117,8 @@ private function writeStyle(): void
         $xmlWriter->writeElementIf($style->isItalic() !== null, 'w:iCs', 'w:val', $this->writeOnOf($style->isItalic()));
 
         // Strikethrough, double strikethrough
-        $xmlWriter->writeElementIf($style->isStrikethrough() !== null, 'w:strike', 'w:val', $this->writeOnOf($style->isStrikethrough()));
-        $xmlWriter->writeElementIf($style->isDoubleStrikethrough() !== null, 'w:dstrike', 'w:val', $this->writeOnOf($style->isDoubleStrikethrough()));
+        $xmlWriter->writeElementIf($style->isStrikethrough(), 'w:strike', 'w:val', $this->writeOnOf($style->isStrikethrough()));
+        $xmlWriter->writeElementIf($style->isDoubleStrikethrough(), 'w:dstrike', 'w:val', $this->writeOnOf($style->isDoubleStrikethrough()));
 
         // Small caps, all caps
         $xmlWriter->writeElementIf($style->isSmallCaps() !== null, 'w:smallCaps', 'w:val', $this->writeOnOf($style->isSmallCaps()));