Spritelab set background as block backwards compatibility

[JillianK]

Adds any sprites from the default sprite list that are missing from a project's initial sprites (generally because the defaultSprites have changed since its creation)
Also adds extra logic to check for missing sprites after filtering by category to prevent the new sprite block and the background block from breaking.

This is necessary before we add the block to the toolbox.

Links

• Slack conversation about the issue
• PR that introduced this potential problem

Testing story

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked