Validation should pass None through

[thclark]

Calling:

name='whatever_strand'
validate_strand(self, name, source=None)

in the case where the twine has no strand name results in a KeyError, because the implementation assumes presence of the key, but the validate method does not have an else statement in the case that if strand_data is not None:

def validate_strand(self, name, source, **kwargs):
    """ Validates a single strand by name
    """
    return self.validate({name: source}, **kwargs)[name]

In general if asked to validate a None strand, Twined should pass the None through.

To fix:

            if strand_data is not None:
                # ...
            else:
                sources[strand_name] = None

[thclark]

Workaround looks like this:
Workaround does NOT look like this because validation exceptions in Twined (rightly) also inherit from KeyError 😭

            # Working around https://github.com/octue/twined/issues/55
            try:
                serialised[k] = self.twine.validate_strand(k, getattr(self, k), cls=OctueJSONEncoder)
            except KeyError as e:
                print(e.args[0])
                serialised[k] = None

[thclark]

Closed by #53