runners: jlink: Fix NoneType object error

The commit 221199e1 presents a bug that
makes west flash failed with error.

AttributeError: 'NoneType' object has no attribute 'startswith'

In function is_tunnel(), tunnel may contain None and has no attribute
"startswith". Fix it.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>