bugfix: error when exporting entries with LinkFields referencing an entry that does not exist on the default site

[rsisson]

To replicate the issue:

• Create a multisite Craft install & install migration assistant
• Create a secondary site
• Create a section with Propagation Method set to "Let each entry choose which sites it should be saved to" and add it to your secondary site (make sure the entry type includes a LinkField)
• Create two entries in your new section, make sure one of them links to the other one and each entry is only enabled on the secondary site
• Attempt to create a migration for the entry with the populated LinkField

Expected outcome: A migration is successfully created Actual outcome: 500 error, processing fails at line 337 of BaseContentMigration::getSourceHandle() due to $element being null

Proposed solution: explicitly pass the site ID of the parent element into Craft::$app->elements->getElementById() at BaseContentMigration::getFieldContent() on line 103