Bug: Fallback system fonts render as square if the fallback fonts are not included in the pubspec

[MaximeRougieux]

Minimum reproduction : https://github.com/MaximeRougieux/repro_adaptive_test_datepicker_fonts

Cupertino and Material widgets use fonts that the developer might not have added in their pubspec, and thus they don't appear in the manifest and are not picked up by loadFonts.

Goldens then renders as this :