Generators: add tests covering handling of invalid docs

[jrfnl]

Description

This adds another set of dedicated tests to safeguard how XML docs which don't follow the specification are handled.

This initial set of tests for this documents the current behaviour [*]. This behaviour may not always be the desired behaviour, in which case, this will be fixed in follow-up commits.

To get these tests up and running, the following fatal errors needed to be fixed:

• Fatal error when a code comparison only contains a single code element:

  Fatal error: Uncaught Error: Call to a member function getAttribute() on null in path/to/PHP_CodeSniffer/src/Generators/Markdown.php:253
  Stack trace:
  #0 path/to/PHP_CodeSniffer/src/Generators/Markdown.php(139): PHP_CodeSniffer\Generators\Markdown->getFormattedCodeComparisonBlock(Object(DOMElement))
  #1 path/to/PHP_CodeSniffer/src/Generators/Markdown.php(39): PHP_CodeSniffer\Generators\Markdown->processSniff(Object(DOMElement))
  #2 path/to/PHP_CodeSniffer/src/Runner.php(99): PHP_CodeSniffer\Generators\Markdown->generate()
  #3 path/to/PHP_CodeSniffer/bin/phpcs(14): PHP_CodeSniffer\Runner->runPHPCS()
  #4 {main}
    thrown in path/to/PHP_CodeSniffer/src/Generators/Markdown.php on line 253
  

• Fatal error when a code element contains no textual content:

  Fatal error: Uncaught Error: Call to a member function getAttribute() on null in path/to/PHP_CodeSniffer/src/Generators/Markdown.php:246
  Stack trace:
  #0 path/to/PHP_CodeSniffer/src/Generators/Markdown.php(139): PHP_CodeSniffer\Generators\Markdown->getFormattedCodeComparisonBlock(Object(DOMElement))
  #1 path/to/PHP_CodeSniffer/src/Generators/Markdown.php(39): PHP_CodeSniffer\Generators\Markdown->processSniff(Object(DOMElement))
  #2 path/to/PHP_CodeSniffer/src/Runner.php(99): PHP_CodeSniffer\Generators\Markdown->generate()
  #3 path/to/PHP_CodeSniffer/bin/phpcs(14): PHP_CodeSniffer\Runner->runPHPCS()
  #4 {main}
    thrown in path/to/PHP_CodeSniffer/src/Generators/Markdown.php on line 246
  

Both of these fatals are fixed by adding defensive coding validating that there are (at least) two code blocks to the getFormattedCodeComparisonBlock() methods for all three generator classes.

Suggested changelog entry

Generators: prevent fatal error when the XML documentation does not comply with the requirements

Related issues/external references

This PR is part of a series of PRs which will add a complete set of tests (and improvements) for the Generator feature.

Also see: #671 and other PR with the Core Component: Generators label.