[iOS] Fallback to default value correctly for optional preferences

[kylehickinson]

For preferences with optional types, the container check was incorrectly casting to the full ValueType which itself was already optional instead of the optionally-erased V type which would cause a successful cast to an Optional<V>.some(nil) rather than to a proper nil value

Resolves brave/brave-browser#42320

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[brave-builds]

Released in v1.75.29

[kjozwiak]

Verification PASSED on iPad Air (3rd Gen) running iOS 18.1 using the following build(s):

Brave | 1.75.29 Chromium: 131.0.6778.69 (Official Build) nightly (64-bit)
--- | ---
Revision | 5107c49b054e241d3dd2eac141c4246b180b4c94
OS | iOS

Using the STR/Cases outlined via brave/brave-browser#42320 (comment), reproduced the original issue by installing 1.71.82 and then upgrading to 1.75.23 as per the following:

ScreenRecording_11-18-2024.23-20-18_1.MP4

Using the same STR/Cases mentioned above, ensured that the issue wasn't occurring when installing 1.71.82 and then upgrading to 1.75.29 as per the following:

ScreenRecording_11-18-2024.23-25-15_1.MP4