[#2123] Fix zoom bug if zUser is undefined

[jonasongg]

Fixes #2123

Proposed commit message

Fix error thrown when zUser is undefined

Currently, an error is thrown when zUser is defined in c-zoom.vue. This
might be caused by zFilterSearch containing a query that returns no
results. Though this happening through normal user interaction is rare
(editing URL, for example), RepoSense should handle this case
gracefully rather than throwing an error.

Let's fix this bug by resetting the zoom view if the error happens.

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