[backport v2.8.next1] Custom banner header without fontSize breaks top navigation

[github-actions]

This is a backport issue for #10140, automatically created via GitHub Actions workflow initiated by @richard-cox

Original issue body:

Setup

• Rancher version: v2.8.0
• Rancher UI Extensions: None
• Browser type & version: Chrome

Describe the bug
In Rancher 2.7.9, if you customized your UI to have a custom header banner, the fontSize is not a required field. This worked fine and it fell back to the default of 14px.

In Rancher 2.8.0, fontSize not being set will result in the entire Vue component failing to render, breaking navigation.

The error comes from this line: https://github.com/rancher/dashboard/blob/master/shell/components/nav/TopLevelMenu.vue#L79
which will be undefined if the fontSize is not set.

To Reproduce
Set a custom header banner without a fontSize defined.

Result
Your navigation will no longer render and you will see console errors.

Expected Result
The navbar should work with a fallback default fontSize

[richard-cox]

Workaround details in #10368 (comment)

[richard-cox]

to reproduce bug #10369 (comment)

[IsaSih]

Tests pass