Theme is not Applied When No One is On-Call

[mgundel]

Describe the bug
Themed styling is not applied to "on-call user" text when an on-call user is not found for a schedule, or a schedule itself is not found.

To Reproduce
Steps to reproduce the behavior:

1. Navigate to an invalid schedule with a theme applied, such as v1/schedules/F.html?theme=grafana

Expected behavior
The text within the user_name div is properly styled.

[sergiitk]

Thanks for the contribution!