Allow theme without second topbar menu

[marc-farre]

Is your feature request related to a problem? Please describe.

1. Create a theme without #topbar-second element, e.g. the Clean Theme.
2. In the console, you'll have Uncaught TypeError: Cannot read properties of undefined (reading 'offsetHeight')

The problem is because of $topBarSecond[0].offsetHeight here: https://github.com/humhub/humhub/blob/master/static/js/humhub/humhub.ui.topNavigation.js#L52C42-L52C71

Describe the solution you'd like

Test if #topbar-second exists before reading it with JS.

[marc-farre]

@luke- would you accept a PR where I add tests in humhub.ui.topNavigation.js and humhub.ui.view.js?
Or should I add an empty <div id="topbar-second"></div> in the theme?

[luke-]

@marc-farre Yes, would be fine for me. So feel free to create a PR which tests if the second toolbar exists.

[marc-farre]

@luke- PR #6728

I've changed the way of calculating the topbar menus height: https://github.com/humhub/humhub/pull/6728/files#diff-b154946f00749e44d107396a9627ecd5987a0bab499487d466968f514df4bdc3R163-R168

Before: Position of the second menu from the top + height of the second menu
After: sum of the heights of all top menus (based on .topmenu class)

[luke-]

@marc-farre Looks fine for me. Thanks!