fix(caldav): do not crash with error 400 when fetching the list of all projects

Because the "all projects" handler is the same as the one to fetch a single project, the handler would fail because no project was specified. However, it should return an empty project instead so that it later fetches all projects.

Resolves https://community.vikunja.io/t/http-400-when-trying-to-connect-via-caldav/3054

pkg/routes/caldav/handler.go

@@ -172,7 +172,7 @@ func EntryHandler(c echo.Context) error {
 func getProjectFromParam(c echo.Context) (project *models.ProjectWithTasksAndBuckets, err error) {
 	param := c.Param("project")
 	if param == "" {
-		return nil, echo.ErrBadRequest
+		return &models.ProjectWithTasksAndBuckets{}, nil
 	}
 
 	s := db.NewSession()