Allow WIllPopCallback to return null or false to veto the pop.

[darrenaustin]

Description

Currently any application using a WillPopCallback to veto or allow a pop to take place is required to either return true or false. However, a common use case for using the willPop mechanism in routes would be to use the return value from an AlertDialog that is asking the user whether they wanted to continue or not. When a dialog is dismissed by tapping on the background barrier the return value will from showDialog will be null which will cause the willPop mechanism to fail with an exception.

This PR relaxes the interpretation of the return value from WillPopCallback to treat null the same as false (i.e. veto the pop operation). It will only allow the pop if the callback returns true.

Related Issues

Fixes: #51768

Tests

I added a new test that checks for both null and false and ensures they are treated the same.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
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• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

Did any tests fail when you ran them?

• No, no existing tests failed, so this is not a breaking change.

[gspencergoog]

[chunhtai]

LGTM