Refactor FXIOS-10205 [Swiftlint] Resolve 1 implicitly_unwrapped_optional violations in WallpaperSelectorViewController

[tonell-m]

📜 Tickets

Jira ticket
Github issue

💡 Description

Continuing resolving implicitly_unwrapped_optional violations in order to enable the rule for further developments.

📝 Checklist

You have to check all boxes before merging

• Filled in the above information (tickets numbers and description of your work)
• Updated the PR name to follow our PR naming guidelines
• Wrote unit tests and/or ensured the tests suite is passing
• When working on UI, I checked and implemented accessibility (minimum Dynamic Text and VoiceOver)
• If needed, I updated documentation / comments for complex code and public methods
• If needed, added a backport comment (example @Mergifyio backport release/v120)