Remove call to SkTypeface::MakeDefault (flutter#47344)

Skia plans to remove the default (global) SkFontMgr, which in turn,
requires the removal of a default typeface (e.g. the
typeface returned by `SkTypeface::MakeDefault` and when `nullptr` passed
in to the `SkFont` constructor).

See https://g-issues.skia.org/issues/305780908

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shell/common/serialization_callbacks.cc

@@ -22,7 +22,7 @@ sk_sp<SkData> SerializeTypefaceWithData(SkTypeface* typeface, void* ctx) {
 sk_sp<SkTypeface> DeserializeTypefaceWithoutData(const void* data,
                                                  size_t length,
                                                  void* ctx) {
-  return SkTypeface::MakeDefault();
+  return nullptr;
 }
 
 struct ImageMetaData {