applets/systemmonitor: handle null faceController

Reason for the change

When initially adding a systemmonitor widget to a panel, faceController can still be null, causing several TypeErrors.

Instead, use optional chaining and default values to prevent these errors until the faceController is loaded.

Test plan

1. Add a system monitor widget
2. (without) TypeErrors (with) no TypeErrors

Screenshots or screen recordings

N/A

Bugs fixed

N/A