[BUGFIX] Fix extension scanner Index.rst filename parsing

Rst files may not contain issue numbers. The extension scanner
iterates all .rst files (including Index.rst) which cases a type
error for typo3/sysext/core/Documentation/Changelog/7.0/Index.rst

(1/1) TypeError
Return value of
TYPO3\CMS\Install\UpgradeAnalysis\DocumentationFile::parseIssueId()
must be of the type string, null returned

Releases: master, 9.5
Resolves: #91027
Related: #90923
Change-Id: I000675700f9da17f063ccbc52d6ce0beabebf247
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64161


Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>