Avoid warning if variable is null

Author: troosan

src/PhpWord/Writer/ODText/Style/Table.php

@@ -47,7 +47,7 @@ public function write()
         $xmlWriter->endElement(); // style:style
 
         $cellWidths = $style->getColumnWidths();
-        $countCellWidths = count($cellWidths);
+        $countCellWidths = $cellWidths === null ? 0 : count($cellWidths);
 
         for ($i = 0; $i < $countCellWidths; $i++) {
             $width = $cellWidths[$i];