Fix all isset.variable phpstan violations

Problem/Motivation

Follow up from #3565604: Remove an unnecessary isset() from Renderer::doRender()

PHPStan level 1 reports 47 isset.variable violations.

Variable $foo in isset() always exists and is not nullable.

I suspect they are all fairly simple to fix.

Steps to reproduce

Violation count:

$ grep 'isset.variable' core/.phpstan-baseline.php | wc -l
47

Violation list:

grep 'isset.variable' core/.phpstan-baseline.php -B2 -A3

But this doesn't give you the actual line numbers, that requires removing the violations from the baseline and rescanning:

# Clear the baseline
$ rm core/.phpstan-baseline.php && echo "<?php return [];" > core/.phpstan-baseline.php
# Run phpstan and grep for the error
$ ./vendor/bin/phpstan analyse -c core/phpstan.neon.dist --error-format=raw \
  | sed "s|$PWD|.|g" \
  | grep -F 'isset() always exists and is not nullable'
# Reset the baseline
$ git checkout main core/.phpstan-baseline.php

I've attached the output to this issue.

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet