MT#64121 fix $c->error $c->error handling

* errors coming from Utils::Preferences can have
  only 1 element in $c->error, the case is now correctly
  handled.

Change-Id: I9f5d384da8b5cb50b78636309dbaab5efa9bdee2
(cherry picked from commit 9d70f62f70)
(cherry picked from commit 56825b26cc)

lib/NGCP/Panel/Role/API.pm

@@ -1069,7 +1069,7 @@ sub log_response {
         $has_errors = 1;
 
         $errors = $c->stash->{is_api_error_response}
-                    ? join ', ', splice @{$c->error}, 1
+                    ? join ', ', (splice @{$c->error}, 1) // ()
                     : join ', ', @{$c->error};
 
         # unhandled error message, should return 500 instead of 200