azureblob/b2/dropbox/gcs/koofr/qingstor/s3: fix 0 length files

In 0386d22cc9 we introduced a test for 0 length files read the
way mount does.

This test failed on these backends which we fix up here.

backend/b2/b2.go

@@ -1531,6 +1531,7 @@ var _ io.ReadCloser = &openFile{}
 
 // Open an object for read
 func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.ReadCloser, err error) {
+	fs.FixRangeOption(options, o.size)
 	opts := rest.Opts{
 		Method:  "GET",
 		Options: options,