Update ClassPluginPhp.php (fix #103)

[convenient]

See https://github.com/AmpersandHQ/ampersand-magento2-upgrade-patch-helper/issues/103#issue-1796759590

Handle cases where the plugin definition has not been fully formed.

Without this change you get a failure like

Test 'FunctionalTests::testMagentoTwoFourFivePointOneNoDb' started
In ClassPluginPhp.php line 78:

  Undefined array key "instance"  

analyse [-a|--auto-theme-update [AUTO-THEME-UPDATE]] [--sort-by-type] [--phpstorm-threeway-diff-commands] [--vendor-namespaces [VENDOR-NAMESPACES]] [--filter FILTER] [--pad-table-columns PAD-TABLE-COLUMNS] [--php-strict-errors] [--show-info] [--] <project>
Test 'FunctionalTests::testMagentoTwoFourFivePointOneNoDb' ended

Checklist

• [ ] Pull request has a meaningful description of its purpose
• [ ] All commits are accompanied by meaningful commit messages
• [ ] Tests have been ran / updated