runners: jlink: Fix NoneType object error

The commit 221199e 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>

scripts/west_commands/runners/jlink.py

@@ -37,7 +37,7 @@ def is_ip(ip):
     return True
 
 def is_tunnel(tunnel):
-    return tunnel.startswith("tunnel:")
+    return tunnel.startswith("tunnel:") if tunnel else False
 
 class ToggleAction(argparse.Action):