Treat external predicate functions that return None as passing in expect()

[dgilland]

Consider functions that return None as having passed validation.

This is to mainly support assert style predicate functions:

def validate(value):
    assert value

# This currently throws an AssertionError.
expect(True, validate)

Currently, the only way these type of functions pass is if they return truthy:

def validate(value):
    assert value
    return True