Fix method_exists() PHP 8 error

[clayliddell]

This ensures that the indexer service exists before attempting to check whether it has a specific method.

• Test coverage exists
• Documentation exists

QA Steps

This will need to be tested on a site running php 8.

• Navigate to /api/1
• Ensure you do not see the following error:

TypeError: method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given in method_exists() (line 307 of /mnt/www/html/datamedicaiddev/docroot/modules/contrib/dkan/modules/common/src/Storage/AbstractDatabaseTable.php) #0 /mnt/www/html/datamedicaiddev/docroot/modules/contrib/dkan/modules/common/src/Storage/AbstractDatabaseTable.php(307): method_exists(NULL, 'modifySchema')
#1 /mnt/www/html/datamedicaiddev/docroot/modules/contrib/dkan/modules/common/src/Storage/AbstractDatabaseTable.php(273): Drupal\common\Storage\AbstractDatabaseTable->tableCreate('jobstore_drupal...', Array)
#2 /mnt/www/html/datamedicaiddev/docroot/modules/contrib/dkan/modules/common/src/Storage/AbstractDatabaseTable.php(83): Drupal\common\Storage\AbstractDatabaseTable->setTable()