Make backingStore optional rather than force unwrapped

[alessdiimperio]

New Pull Request Checklist

• [ x] I am not disclosing a vulnerability.
• [ x] I am creating this PR in reference to an issue.

Issue Description

While testing one perhaps doesn't want to initialize Parse or all other dependencies. however Installation.current is optional but it's backingStore is force unwrapped causing a crash when trying to get .current installation if Parse hasn't been initialized rather than just returning nil.

Closes: issue

Approach

removed ! added ? made it compile 🤷

TODOs before merging

• [ /n] Add tests
• [ /n] Add entry to changelog
• [ /n] Add changes to documentation (guides, repository pages, in-code descriptions)

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