fix(bug): Check for the cms background before trying to access it

[vbudhram]

Because

• This causes and unexpected error

This pull request

• Check for the background before trying to access it

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12391

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

This will need to be uplifted to stage.

[Copilot]

Pull Request Overview

This PR adds defensive null checking for CMS background configuration to prevent unexpected errors when accessing background-related properties. The fix ensures that backgrounds property exists before attempting to access nested background configuration values.

• Added null checks for this.cmsConfig.shared.backgrounds property before accessing defaultLayout
• Applied the same defensive check in two locations where background configuration is accessed

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[vpomerleau]

Why not this.cmsConfig?.shared?.backgrounds?.defaultLayout?

[vbudhram]

No reason, just followed the existing pattern.