ulozto: fix panic in various integration tests

Before this change some of the integration tests were producing this error

    panic: runtime error: invalid memory address or nil pointer dereference

This was caused by an `fs.Object` of which the type (`*Object`) was
not `nil`, but the value within was `nil`. These do not compare as
`nil` leading to the panic.

This is a classic Go gotcha: https://go.dev/doc/faq#nil_error

This was easily fixed by changing the type of one function to return
fs.Object instead of *Object.

backend/ulozto/ulozto.go

@@ -1020,7 +1020,7 @@ func (f *Fs) CreateDir(ctx context.Context, parentSlug, leaf string) (newID stri
 	return folder.Slug, nil
 }
 
-func (f *Fs) newObjectWithInfo(ctx context.Context, remote string, info *api.File) (*Object, error) {
+func (f *Fs) newObjectWithInfo(ctx context.Context, remote string, info *api.File) (fs.Object, error) {
 	o := &Object{
 		fs:     f,
 		remote: remote,