The old flags used a size of 150x100, while the new assets based on
twemoji use a size of 150x108. Update existing usages to match this new
aspect ratio better.
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.