[pigeon] Use hasLength and isEmpty in tests for better failure messages

[srawlins]

When an expectation like expect(foo.length, equals(0)) fails, you just see a message like "7 is not 0." Changing these tests to use isEmpty and hasLength, like expect(foo, isEmpty), you'll instead see a much more useful message like "expected this thing to have 0 elements, but instead it was [a, b, c]."

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request updates tests in packages/pigeon/test/pigeon_lib_test.dart to use dedicated collection matchers like isEmpty, hasLength, and containsAll. These changes improve the readability and failure output of the tests.

[srawlins]

This is test-excempt as it only affects tests, and I believe it is also version-bump-exempt for the same reason.