Fix a crash on incomplete/missing threads

Properly handle situation where we can get a None as well as an empty
message list.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/b4/mbox.py b/b4/mbox.py
index 850558f..724e956 100644
--- a/b4/mbox.py
+++ b/b4/mbox.py
@@ -501,7 +501,7 @@ def main(cmdargs):
             sys.exit(1)
 
         msgs = b4.get_pi_thread_by_msgid(msgid, useproject=cmdargs.useproject, nocache=cmdargs.nocache)
-        if not len(msgs):
+        if not msgs:
             return
     else:
         if cmdargs.localmbox == '-':