Set App.app to initial value of None to avoid AttributeError in teardown

[HalfWhitt]

Running the entire core test suite completes successfully. But at least one test (and probably others), when run individually, fail. Running tests/style/pack/layout/test_rtl.py by itself gets six instances of this:

    @pytest.fixture(autouse=True)
    def no_dangling_tasks():
        """Ensure any tasks for the test were removed when the test finished."""
        yield
>       if toga.App.app:
E       AttributeError: type object 'App' has no attribute 'app'

tests/conftest.py:22: AttributeError

This is fine if a previous test has created an app instance, which is why it only fails in isolation. Changing it to a hasattr check fixes it, but then it still needs an additional truthiness or None check to avoid failing test_window_created_without_app, which sets App.app to None. At that point, why not just set None as the initial value in the class definition? It makes semantic sense for it to be None if there's no running app.

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

[freakboy3742]

Seems entirely reasonable. Thanks for the fix!