Issue #3517125: Fix coding standard (PHPStan) issues and test failures

Closes #3517125

src/Export/ExportPluginBase.php

@@ -190,7 +190,7 @@ abstract class ExportPluginBase extends PluginBase implements ExportPluginInterf
   /**
    * {@inheritdoc}
    */
-  public function setLogger(LoggerInterface $logger = NULL): void {
+  public function setLogger(?LoggerInterface $logger = NULL): void {
     $this->logger = $logger;
   }
 
@@ -773,7 +773,7 @@ abstract class ExportPluginBase extends PluginBase implements ExportPluginInterf
    * @return string
    *   The ID of a content entity migration plugin definition.
    */
-  private function getMigrationId(string $entity_type_id, string $bundle = NULL): string {
+  private function getMigrationId(string $entity_type_id, ?string $bundle = NULL): string {
     return implode('_', array_filter([
       $this->configuration['id-prefix'],
       $entity_type_id,
@@ -836,7 +836,7 @@ abstract class ExportPluginBase extends PluginBase implements ExportPluginInterf
    *   The current progress. If this is NULL, then the cache will be dropped
    *   without any condition.
    */
-  protected function flushEntityMemoryCache(int $progress = NULL): void {
+  protected function flushEntityMemoryCache(?int $progress = NULL): void {
     if (
       $progress === NULL ||
       fmod((float) $progress, 10.0) < 0.01