[ffigen] Fix some minor bugs

[liamappelbe]

Fix 2 small issues that came up during the big config refactor.

• C function signatures with an empty arg list should codegen as (void) instead of ()
• There were cases where we were using @Native symbol loads without setting @DefaultAsset

Bumping to 22.0.0-wip too since I'm about to do that anyway.

[github-actions]

Package publishing

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Package	Version	Status	Publish tag (post-merge)
package:code_assets	1.3.0-wip	WIP (no publish necessary)
package:data_assets	0.20.0	already published at pub.dev
package:ffi	2.2.0	already published at pub.dev
package:hooks	2.2.0-wip	WIP (no publish necessary)
package:hooks_runner	1.6.2-wip	WIP (no publish necessary)
package:jni_flutter	1.0.2	already published at pub.dev
package:jni_util	1.0.0	already published at pub.dev
package:native_toolchain_c	0.19.4-wip	WIP (no publish necessary)
package:record_use	1.1.0	ready to publish	record_use-v1.1.0
package:swift2objc	0.3.0-wip	WIP (no publish necessary)
package:swiftgen	0.1.3	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:objective_c	pkgs/objective_c/lib/src/objective_c_bindings_generated.dart pkgs/objective_c/pubspec.yaml

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	None	9.5.0	9.5.0	9.5.0	✔️

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[goderbauer]

LGTM

[goderbauer]

Can we cover this change with a test?