Xls Writer and Empty RichText

Fix #918, which went stale in 2019 and is now reopened. Writing out a RichText object with no text to Xls creates a file which Excel considers corrupt. Treating such an object as a null string rather than a RichText object when writing eliminates the problem.

src/PhpSpreadsheet/Writer/Xls.php

@@ -150,6 +150,9 @@ class Xls extends BaseWriter
                 /** @var Cell $cell */
                 $cell = $this->writerWorksheets[$i]->phpSheet->getCellCollection()->get($coordinate);
                 $cVal = $cell->getValue();
+                if ($cVal instanceof RichText && (string) $cVal === '') {
+                    $cVal = '';
+                }
                 if ($cVal instanceof RichText) {
                     $active = $this->spreadsheet->getActiveSheetIndex();
                     $sheet = $cell->getWorksheet();

src/PhpSpreadsheet/Writer/Xls/Worksheet.php

@@ -369,6 +369,9 @@ class Worksheet extends BIFFwriter
             $xfIndex = $cell->getXfIndex() + 15; // there are 15 cell style Xfs
 
             $cVal = $cell->getValue();
+            if ($cVal instanceof RichText && (string) $cVal === '') {
+                $cVal = '';
+            }
             if ($cVal instanceof RichText) {
                 $arrcRun = [];
                 $str_pos = 0;

tests/PhpSpreadsheetTests/Writer/Xls/Issue918Test.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Writer\Xls;
+
+use PhpOffice\PhpSpreadsheet\Helper\Html as HtmlHelper;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PhpOffice\PhpSpreadsheetTests\Functional\AbstractFunctional;
+
+class Issue918Test extends AbstractFunctional
+{
+    public function testEmptyRichText(): void
+    {
+        // Issue 918 - Xls Writer creates corrupt file with empty RichText.
+        $spreadsheet = new Spreadsheet();
+        $sheet = $spreadsheet->getActiveSheet();
+        $helper = new HtmlHelper();
+        $html = '<div></div>';
+        $richValue = $helper->toRichTextObject($html);
+        self::assertCount(0, $richValue->getRichTextElements());
+        $sheet->getCell('A1')->setValue($richValue);
+
+        $robj = $this->writeAndReload($spreadsheet, 'Xls');
+        $spreadsheet->disconnectWorksheets();
+        $sheet0 = $robj->getActiveSheet();
+        self::assertNull($sheet0->getCell('A1')->getValue(), 'empty text object has been changed to null');
+        $robj->disconnectWorksheets();
+    }
+}