Bug: CPU plugin label can't be unset/blanked

### Describe the bug
Setting `@dracula-cpu-usage-label` to `""` fails to unset CPU plugin's label.
Also unsetting the variable with `set -gu @dracula-cpu-usage-label` does not work.

### To Reproduce
Steps to reproduce the behavior:
1. `set -g @dracula-cpu-usage-label ""`
2. Reload `tmux`
3. Label is set to default `CPU`.

### Expected behavior
Label should be blankable to save statusbar estate. I guess that the current default when blank is not preserving previous behavior.

### System
- OS: Ubuntu Bionic
- Tmux Version: 2.6