Remove <MISSING_DATA> from input_values

Hi,

A Task provides optional input values.

But if you get the self.input_values you can find such thing.

{..., 'sleep_time': <MISSING_DATA>}

This means if you use this dict to retrieve this input value, you retrieve a string <MISSING_DATA> (or an object stringified as <MISSING_DATA>)

assert "sleep_time" not in self.input_values              # this will fail
assert self.input_values.get("sleep_time", None) is None  # this will fail

It's maybe easier to just not expose such keys from the input_values dict?