fix(coredata): make Item optional on SavedItem

[cyndichin]

Summary

Set Item back to non-optional on SavedItem since there are scenarios such as extension and offline where we can't create an Item (which is server backed)

References

See convo here:
https://pocket.slack.com/archives/C01UH57EJKD/p1683046168112139?thread_ts=1683042594.907889&cid=C01UH57EJKD

Implementation Details

• Make Item optional on SavedItem
• Fix build errors for compiling project and running tests

Test Steps

1. Save an article to Pocket via Share Extension on airplane mode
2. Navigate to Saves List
3. Validate that no crash occurs

PR Checklist:

• N / A Added Unit / UI tests
• Self Review (review, clean up, documentation, run tests)
• Basic Self QA

Screenshots

[pocket-ci]

	Messages
📖	No SwiftLint violations! 🎉
📖	Project coverage: 76.57%
📖	Checking XCode Environment Variables
📖	Edited 16 files
📖	Created 0 files

PocketKit: Coverage: 76.76

File	Coverage
SavedItem+ItemsListItem.swift	78.43%	✅
SavedItem+Listen.swift	0.0%	⚠️
SavedItemsListViewModel.swift	83.59%	✅
Item+Extensions.swift	88.24%	✅
ListenViewModel.swift	0.0%	⚠️
HomeViewModel.swift	78.14%	✅
SavedItemViewModel.swift	86.21%	✅

Sync: Coverage: 84.51

File	Coverage
PocketSource.swift	84.9%	✅
SavedItem+CoreDataProperties.swift	100.0%	✅
CoreDataSpotlightDelegate.swift	93.1%	✅

Generated by 🚫 Danger Swift against e7e73a0