lint: clean up len-as-condition warnings

Rather than count iterables just to test equality to 0, use their
implicit truthiness instead.  Shouldn't be any functional changes.

BUG=chromium:980619
TEST=`cros lint` is clean w/pylint-1.7 again

Change-Id: I9cf9ca1ed7ff90d540e9d93f777562b4103afc1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1818779
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index ca4e53d..40397f6 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -243,7 +243,7 @@
   if working_dir is not None:
     cmd.extend(['--working_dir', working_dir])
 
-  if len(additional_args) > 0:
+  if additional_args:
     cmd.append('--')
     cmd.extend(additional_args)