Always sleep between iterations of generator handlers

[rmartin16]

Changes

• Previously, generator handlers would only sleep if the iteration's return value was truthy
• Now, asyncio.sleep() is always invoked for each generator iteration

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

CI testbed failure, fyi:

FAILED tests/widgets/test_webview.py::test_load_url - RuntimeError: Error Domain=WKErrorDomain Code=4 "A JavaScript exception occurred" UserInfo={WKJavaScriptExceptionLineNumber=1, WKJavaScriptExceptionMessage=TypeError: null is not an object (evaluating 'document.body.innerHTML'), WKJavaScriptExceptionColumnNumber=14, WKJavaScriptExceptionSourceURL=https://github.com/beeware, NSLocalizedDescription=A JavaScript exception occurred}

https://github.com/beeware/toga/actions/runs/10688375651/job/29628280415#step:6:768

[freakboy3742]

👍

This is one of those "I know this has been here forever, but it's never annoyed me enough to fix it" bugs. Thanks :-)