Writer: Xlsx: Worksheet: handle PHP 7.4 deprecation notice

Author: kornrunner

src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

@@ -1105,7 +1105,7 @@ private function writeCell(XMLWriter $objWriter, PhpspreadsheetWorksheet $pSheet
                     break;
                 case 'f':            // Formula
                     $attributes = $pCell->getFormulaAttributes();
-                    if ($attributes['t'] === 'array') {
+                    if (($attributes['t'] ?? null) === 'array') {
                         $objWriter->startElement('f');
                         $objWriter->writeAttribute('t', 'array');
                         $objWriter->writeAttribute('ref', $pCellAddress);