fix: return 404 for empty repositories (#7003)

- Some endpoints (`/api/v1/repos/*/*/raw`, `/api/v1/repos/*/*/media`, ...;
anything that uses both `context.ReferencesGitRepo()` and
`context.RepoRefForAPI` really) returned a 500 when the repository was
completely empty. This resulted in some confusion in
https://github.com/datalad/datalad-usage-dashboard/issues/47 because the
same request for a non-existent file in a repository could sometimes
generate a 404 and sometimes a 500, depending on if the git repository
is initialized at all or not.

Returning a 404 is more appropriate here, since this isn't an
unexpected internal error, but just another way of not finding the
requested data.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7003
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Matthias Riße <m.risse@fz-juelich.de>
Co-committed-by: Matthias Riße <m.risse@fz-juelich.de>

services/context/api.go

@@ -364,6 +364,11 @@ func RepoRefForAPI(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		ctx := GetAPIContext(req)
 
+		if ctx.Repo.Repository.IsEmpty {
+			ctx.NotFound(fmt.Errorf("repository is empty"))
+			return
+		}
+
 		if ctx.Repo.GitRepo == nil {
 			ctx.InternalServerError(fmt.Errorf("no open git repo"))
 			return