Fix Hint.coerce_type not recognising falsey statements

[Ultrasonic1209]

Describe what bug you fixed
This PR fixes Hint.coerce_type so that it correctly recognises falsey statements.

Checklist

• I've already read the contributing guide.
• I've run all tests and make sure all exists test cases are passed.
• I've make sure this PR works with Sanic-18.12 and Python-3.5.

[ahopkins]

This still needs some more work to handle

    @dataclass
    class Query:
        maybe: Optional[bool] = None

And particularly when dealing with [None].