fix(platform-browser): take serialization into account for empty check of transfer state

[Jakob-em]

Transfer states with no keys and only serialization callbacks were previously treated as empty which lead to no script getting embedded to transfer the state from the server to the client.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

I am not sure if there is anything in the docs that needs to be updated.

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

When no key is present in the transfer state and serialization callbacks are registered, these callbacks are never called as the state is treated as empty and toJson is therefore not called on the server side.

Issue Number: N/A

What is the new behavior?

Transfer states without keys and serialization callbacks are now treated as non empty which allows them to be send from the server to the client.

Does this PR introduce a breaking change?

• Yes
• No
  Considering the current behaviour being not very logical and probably not intended I would not consider this change a breaking change.

[alan-agius4]

Hi @Jakob-em,

by the looks of it, the issue that you are trying to fix has been reported in #47172. Unfortunately, this is not the right place to tackle this problem. I have a PR in flight that addresses the issue.

None-the-less, thank you for your contribution.

[Jakob-em]

Thanks for the update @alan-agius4. I was not sure if this should be handled here or on the usage side. Glad to see the issue is fixed with your PR 👍🏻

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