chore(generator): workaround nnbd #75

[wasabeef]

What does this change?

Fixed: Issue #75

MEMO
If when forgot to add --no-sound-null-safety argument,

Xcode's output:
↳
    lib/gen/assets.gen.dart:1:1: Error: A library can't opt out of null safety by default, when using sound null safety.
    // @dart = 2.10
    ^^^^^^^^^^^^^^^
    lib/gen/colors.gen.dart:1:1: Error: A library can't opt out of null safety by default, when using sound null safety.
    // @dart = 2.10
    ^^^^^^^^^^^^^^^
    lib/gen/fonts.gen.dart:1:1: Error: A library can't opt out of null safety by default, when using sound null safety.
    // @dart = 2.10

What is the value of this and can you measure success?

• Pass CI

[codecov]

Codecov Report

Merging #83 (f605936) into main (7eac2f4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files          16       16           
  Lines         506      506           
=======================================
  Hits          497      497           
  Misses          9        9           

Impacted Files	Coverage Δ
packages/core/lib/generators/generator_helper.dart	100.00% <ø> (ø)

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