Handle Empty String in SharedStrings

Fix #4063. Fix #1560. Fix #1293. PhpSpreadsheet is not accounting for an empty string in Xlsx sharedStrings.xml.The code which parses it in Reader/Xlsx looks for a `t` or `r` tag descending from `si`, but, in this case, the tag is coded as `<si/>`, with neither t nor r tag descending. An else clause is added to set the string to empty string in this case.

I was surprised that this had not turned up before, and a search through the archives found at least 2 earlier reports from 4 years ago. Those had been marked stale; the stale indicator is removed, and the issues are re-opened, to be closed when this PR is merged.

src/PhpSpreadsheet/Reader/Xlsx.php

@@ -700,6 +700,8 @@ class Xlsx extends BaseReader
                                     $sharedStrings[] = StringHelper::controlCharacterOOXML2PHP((string) $val->t);
                                 } elseif (isset($val->r)) {
                                     $sharedStrings[] = $this->parseRichText($val);
+                                } else {
+                                    $sharedStrings[] = '';
                                 }
                             }
                         }

tests/PhpSpreadsheetTests/Reader/Xlsx/Issue4063Test.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Reader\Xlsx;
+
+use PhpOffice\PhpSpreadsheet\IOFactory;
+use PHPUnit\Framework\TestCase;
+
+class Issue4063Test extends TestCase
+{
+    private static string $testbook = 'tests/data/Reader/XLSX/issue.4063.xlsx';
+
+    public function testSharedStringsWithEmptyString(): void
+    {
+        $spreadsheet = IOFactory::load(self::$testbook);
+        $sheet = $spreadsheet->getActiveSheet();
+        $data = $sheet->toArray(null, true, true, true);
+        $nbsp = "\u{00a0}";
+        self::assertSame(['A' => '226', 'B' => '', 'C' => $nbsp], $data[17]);
+        self::assertSame(['A' => '38873', 'B' => 'gg', 'C' => ' '], $data[22]);
+        $spreadsheet->disconnectWorksheets();
+    }
+}