avoid key errors on environment access (#72620)

changelogs/fragments/better_os_environ_access.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - avoid possible errors accessing os.environ by not assuming existance of variables.

lib/ansible/executor/task_executor.py

@@ -1020,7 +1020,7 @@ def start_connection(play_context, variables, task_uuid):
     Starts the persistent connection
     '''
     candidate_paths = [C.ANSIBLE_CONNECTION_PATH or os.path.dirname(sys.argv[0])]
-    candidate_paths.extend(os.environ['PATH'].split(os.pathsep))
+    candidate_paths.extend(os.environ.get('PATH', '').split(os.pathsep))
     for dirname in candidate_paths:
         ansible_connection = os.path.join(dirname, 'ansible-connection')
         if os.path.isfile(ansible_connection):

lib/ansible/module_utils/basic.py

@@ -2649,8 +2649,12 @@ class AnsibleModule(object):
                 old_env_vals[key] = os.environ.get(key, None)
                 os.environ[key] = val
         if path_prefix:
-            old_env_vals['PATH'] = os.environ['PATH']
-            os.environ['PATH'] = "%s:%s" % (path_prefix, os.environ['PATH'])
+            path = os.environ.get('PATH', '')
+            old_env_vals['PATH'] = path
+            if path:
+                os.environ['PATH'] = "%s:%s" % (path_prefix, path)
+            else:
+                os.environ['PATH'] = path_prefix
 
         # If using test-module.py and explode, the remote lib path will resemble:
         #   /tmp/test_module_scratch/debug_dir/ansible/module_utils/basic.py