Xls Reader Array Offset Null (#2338)

See issue #2315. It is nominally solved by PR #2312, but that PR is completely unsuitable for merging. This one-line change is a replacement for that PR.

As with many problems of this type, it is not clear how how to create a spreadsheet with this sort of harmless corruption in the wild. An example was supplied with the issue, and I have tested manually against it. However, the file is huge and not suitable for a formal unit test. I do not understand BIFF well enough to try and craft a suitable example on my own.

Co-authored-by: Adrien Crivelli <adrien.crivelli@gmail.com>

src/PhpSpreadsheet/Reader/Xls.php

@@ -7017,7 +7017,7 @@ class Xls extends BaseReader
                 // offset: 3; size: 2; one-based index to DEFINEDNAME or EXTERNNAME record
                 $index = self::getUInt2d($formulaData, 3);
                 // assume index is to EXTERNNAME record
-                $data = $this->externalNames[$index - 1]['name'];
+                $data = $this->externalNames[$index - 1]['name'] ?? '';
                 // offset: 5; size: 2; not used
                 break;
             case 0x3A:    //    3d reference to cell