tools: Set `NPU_ID` to null string in run script

The `NPU_ID` defines the NPU to be used and it is defaulted to `U65`
which causes issues with Corstone-300 and Corstone-310 which are built
with `U55` as default NPU. Therefore, setting `NPU_ID` to null string
leads to using `U55` for Corstone-300 and Corstone-310 and `U65` for
Corstone-315 as default NPU respectively.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>