ui_tools/views: Fix IndexError when set_focus called on empty ListBox.

[srujanyadav-hyp]

Fixes a crash that occurs when clearing search results in streams or topics view when the underlying list is empty. The application would raise 'IndexError: Can't set focus, ListBox is empty' when attempting to restore focus position after clearing a search. Added checks before calling set_focus() in 4 locations:

• StreamsView.keypress() CLEAR_SEARCH handler
• TopicsView.keypress() CLEAR_SEARCH handler
• TopicsView.update_topics_list() (2 locations) The fix verifies the list is not empty before attempting to set focus, preventing the crash while maintaining expected behavior when lists contain items.

What does this PR do, and why?

Outstanding aspect(s)

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• Fully fixes #
• Partially fixes issue #
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Visual changes