ARC: west runner: mdb: fix unexpected empty argument pass to MDB

Currently, in case of multi-core target or in case jtag adapter
auto-selectio, we pass unexpected empty argument (empty quotes `''`)
to MDB binary due to argument generation error.

Fix that.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>