Beginning since 2af30f715e, Gitea has a
500 error when visiting a user profile with `?tab=overview` in the URL
when the user doesn't have a `.profile/README.md`, because the backend
code assumes that if the tab is overview a profile repository must
exist, and checks that the repository is nil aren't done.
In this PR I treat `?tab=overview` the same as if no tab is set in the
URL, which corrects this behaviour- Now, when visiting `?tab=overview`,
if `.profile/README.md` exists on that user's profile it'll show that as
it should, otherwise it'll show the repositories tab.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>