fix: URI::getSegment() returns non-existent segment

[kenjis]

Description
Fixes #7246 (and #2962)

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Sorry, by accident I wrote the comment under the issue report: #7246 (comment)

[kenjis]

Since michalsn is opposed, this PR is put on hold.
#7246 (comment)

[michalsn]

I was pointing out the potential problem that this change brings.

If everyone wants to treat it as a bug, I'm fine with that. In my opinion, it will be a bit controversial, but we have already made bigger BC changes in the past.

[kenjis]

I am going to withdraw this BC as it does not seem to be worth much.