Use map syntax to clean up `null` handling in `toJson` functions #1136

Job	Run time
analyzer_and_format; Dart 3.5.0; PKGS: _test_yaml, checked_yaml, example, json_annotation, json_s...	36s
mono_repo self validate	14s
analyzer_and_format; Dart dev; PKGS: _test_yaml, checked_yaml, example, json_annotation, json_ser...	38s
unit_test; Dart 3.5.0; PKG: json_serializable; `dart test --run-skipped -t presubmit-only test/an...	2m 9s
unit_test; Dart 3.5.0; PKGS: _test_yaml, checked_yaml, example, json_serializable; `dart test`	57s
unit_test; Dart 3.5.0; PKG: json_serializable; `dart test --run-skipped -t presubmit-only test/en...	45s
unit_test; Dart 3.5.0; PKG: json_serializable; `dart test -p chrome`	53s
unit_test; Dart dev; PKGS: _test_yaml, checked_yaml, example, json_serializable; `dart test`	48s
unit_test; Dart dev; PKG: json_serializable; `dart test --run-skipped -t presubmit-only test/anno...	1m 58s
unit_test; Dart dev; PKG: json_serializable; `dart test --run-skipped -t presubmit-only test/ensu...	44s
unit_test; Dart dev; PKG: json_serializable; `dart test -p chrome`	1m 4s
ensure_build; Dart dev; PKGS: _test_yaml, checked_yaml, example; `dart test --run-skipped -t pres...	1m 16s
ensure_build; Dart 3.5.0; PKGS: _test_yaml, checked_yaml, example; `dart test --run-skipped -t pr...	1m 15s
	13m 17s

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