[Clipboard] Assert at least one clipboard data variant is provided

[loic-sharma]

This change makes the clipboard require at least one clipboard data variant. Today, this effectively makes ClipboardData.text required as it is the only data variant supported by Clipboard. This is a breaking change.

Part of #121976

Background

Currently the different platforms behave unexpectedly if you set your clipboard text to null:

1. Android and iOS will set your clipboard content to the text "null".
2. Web assumes incorrectly that the text is non-null
3. Linux will return an error if the text is null
4. Windows crashes: Clipboard.setData crashes the app #121976

In other words, customers who set their clipboard to null today are broken. Instead, they should set their clipboard's text to the empty string "".

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[yaakovschectman]

LGTM

[bernaferrari]

I don't like the assert, wouldn't something like "text must not be null" be simpler? There are no other variants right now, it isn't clear.

[goderbauer]

LGTM

[loic-sharma]

@bernaferrari Thanks for the feedback, I agree that the previous assertion message was unclear. This should be addressed now, let me know if you have additional comments!

[bernaferrari]

Maaaaybe I would mention in the comment "eventually with unions this won't be nullable anymore" or something like this, because in a few years when someone implements union in Dart, someone could possibly just search the codebase and update. Maybe.

Just an idea.

[loic-sharma]

@bernaferrari The clipboard can contain different content for different formats.
For example, a clipboard can contain both an image and the link to the image.
The app that pastes the clipboard can choose from these different contents
depending on what it supports. It's not clear that a union would be a good fit
for these kinds of scenarios.

[bernaferrari]

That's fair.. I was thinking text only.

[gspencergoog]

[cbracken]

Looks great - and thanks for the detailed notes on the current broken behaviour when this is null. If you haven't already, make sure this gets added to the release notes.

[bernaferrari]

The name of the branch 😂