Do not retrieve credentials from credential_process when set to empty…

… string

Fixes #3098.

botocore/credentials.py

@@ -994,7 +994,7 @@ def __init__(self, profile_name, load_config, popen=subprocess.Popen):
 
     def load(self):
         credential_process = self._credential_process
-        if credential_process is None:
+        if not credential_process:
             return
 
         creds_dict = self._retrieve_credentials_using(credential_process)