avoid index error on empty list, key being true means its not None nor []

fixes #13009

system/known_hosts.py

@@ -92,8 +92,8 @@ def enforce_state(module, params):
     #Find the ssh-keygen binary
     sshkeygen = module.get_bin_path("ssh-keygen",True)
 
-    #trailing newline in files gets lost, so re-add if necessary
-    if key is not None and key[-1]!='\n':
+    # Trailing newline in files gets lost, so re-add if necessary
+    if key and key[-1] != '\n':
         key+='\n'
 
     if key is None and state != "absent":