What happened (please include outputs or screenshots):
When authentication method is exec and the provider is aws, the command can sometimes return non-zero status without stderr. Below is an example of the output ot config.load_kube_config() in this case:

The SSO session associated with this profile has expired or is otherwise invalid. To refresh this SSO session run aws sso login with the corresponding profile.
ERROR:root:'NoneType' object has no attribute 'strip'

in this case the command returns a 255 return code with stderr: None. This causes ConfigException to not be raised.

Proposing a change to kubernetes/base/config/exec_provider.py as such:

--- a/kubernetes/base/config/exec_provider.py
+++ b/kubernetes/base/config/exec_provider.py
@@ -80,8 +80,8 @@ class ExecProvider(object):
         exit_code = process.wait()
         if exit_code != 0:
             msg = 'exec: process returned %d' % exit_code
-            stderr = stderr.strip()
             if stderr:
+                stderr = stderr.strip()
                 msg += '. %s' % stderr
             raise ConfigException(msg)
         try:

What you expected to happen:

ConfigException should be raised if exec fails

How to reproduce it (as minimally and precisely as possible):
Use exec auth configuration with non-valid SSO creds.

Anything else we need to know?:

Environment:

• Kubernetes version (kubectl version): v1.28.3
• OS (e.g., MacOS 10.13.6): macOS 13.6
• Python version (python --version): 3.11.6
• Python client version (pip list | grep kubernetes): 28.1.0