fix exercise title padding when default label is used

Small tweak.

Also, to enumerate the various cases this code block uses:

A Note without a title:

- `:not([data-label])` yields "Note"
- `[data-label='']`    yields ""
- `[data-label!='']`   yields "Custom"

A note with a title (uses `.ui-has-child-title`):

- `:not([data-label])` yields "Note: Title"
- `[data-label='']`    yields "Title"
- `[data-label!='']`   yields "Custom: Title"

Also, both the label and title need to be styled differently depend on the
  presence of the other (hence the `@has-title` and `@has-label` mixin params).