BLT-5217: Undefined array key "bootstrap" for Inspector when Drupal is not installed

[shelane]

Describe the bug
When a site does not yet have Drupal installed and the inspector is called to check, there is a PHP Warning thrown:

PHP Warning: Undefined array key "bootstrap" in vendor/acquia/blt/src/Robo/Inspector/Inspector.php on line 192

Verbose output:

[debug] Verifying that Drupal is installed...
[info] Running command /var/www/html//vendor/bin/drush @self --uri=* status --format=json --fields=* --root=/var/www/html/****/docroot

Warning: Undefined array key "bootstrap" in /mnt/www/html/****/vendor/acquia/blt/src/Robo/Inspector/Inspector.php on line 192

To Reproduce
Steps to reproduce the behavior, ideally starting from a fresh install of BLT:

1. Have a new site that has been created with an empty database where Drupal has not yet been installed
2. Have a command that calls $this->getInspector()->isDrupalInstalled()
3. See error

Expected behavior
A boolean value of FALSE should be returned when it is called.