Fix crash if test is stopped during SSH execution

Sometimes happens that test is stopped during execution, causing an
internal exception that turns out using a stdout variable initialized
with None. Then this will crash kirk due to unhandled TypeError.

libkirk/ssh.py

@@ -281,7 +281,7 @@ async def run_command(
             cmd = self._create_command(command, cwd, env)
             ret = None
             start_t = 0
-            stdout = None
+            stdout = []
             panic = False
             channel = None
             session = None