Fix: treat tab "overview" as "repositories" in user profiles without readme (#27124)

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>

routers/web/user/profile.go

@@ -77,8 +77,9 @@ func userProfile(ctx *context.Context) {
 
 func prepareUserProfileTabData(ctx *context.Context, showPrivate bool, profileGitRepo *git.Repository, profileReadme *git.Blob) {
 	// if there is a profile readme, default to "overview" page, otherwise, default to "repositories" page
+	// if there is not a profile readme, the overview tab should be treated as the repositories tab
 	tab := ctx.FormString("tab")
-	if tab == "" {
+	if tab == "" || tab == "overview" {
 		if profileReadme != nil {
 			tab = "overview"
 		} else {