Previously such use cases have been returning `null` values just fine since `GetLocalisableString` was returning `LocalisableFormattableString`, and these null values were wrapped in `LocalisableString`s implicitly, therefore not requiring an explicit fallback string and `null` doesn't break anything.
The same tooltip can be used for the rank graph, the play history graph,
and the replay history graph.
The only difference between those three is the displayed label, which
has now been included as part of the `TooltipContent`, rather than
unnecessarily recreating tooltips just for different sprite texts.