Don't silently ignore some partial types

[ddfisher]

After #3024, mypy won't complain when inferring the type of a variable as None. Unfortunately, in some circumstances we drop information about the variable that makes this invalid. For example:

T = TypeVar('T')
def f(*x: T) -> Dict[int, T]: pass
x = None
if bool():
    x = f()
reveal_type(x)  # E: Revealed type is 'builtins.None'

[ddfisher]

See also: testPartiallyInitializedToNoneAndThenToIncompleteType.