Create default label when importing assets if none exists

[marcusmoore]

Currently, if a user attempts to import assets and there isn't a status label they are presented with an unhelpful error message:

Your file import is complete, but we did receive an error. This is usually caused by third-party API throttling from a notification webhook (such as Slack) and would not have interfered with the import itself, but you should confirm this.

This PR allows the import to go through by creating a new "Default" status label with the "Pending" status:

A note is added to the status label that indicates how it was created, "Default status label created by AssetImporter".

[what-the-diff]

PR Summary

• Updated Handling of Status Labels in Asset Importer
  The changes to 'AssetImporter.php' make it more reliable when interacting with Status Labels. The updates prevent potential errors when trying to access information about these labels. It also ensures a status label will always be available, creating a default one if needed.

• Introduced Tests for Asset Import
  To verify that the new changes have improved our Asset Importer, we've added a test suite - 'AssetImportTest.php'. These tests check that our default Status Label is correctly selected and that it reverts to the existing one if no deployable label is around. Plus, it ensures a default label is created when none are available. It helps increase confidence in the changes made.