West runner command doesn't work for em-starterkit

[ruuddw]

debug and flash commands for em-starterkit generate an error:

Using runner: em-starterkit
fatal error: command debug cannot be run in this context: Runner em-starterkit received unknown arguments ['']

parse_known_args() returns a list with an empty string instead of an empty list. No idea if this is ok and the check should be changed, or something else should be fixed...
See:

zephyr/scripts/meta/west/cmd/run_common.py

Line 209 in f3f767a

if unknown:

[nashif]

@mbolivar ping