Fix Link investment

[jnnr]

When running datapackages with expandable Links, _nominal_value returns None, but in Link, a dictionary is expected. This gives the following error:

  File "oemof/tabular/datapackage/reading.py", line 408, in deserialize_energy_system
    resource,
  File "oemof/tabular/datapackage/reading.py", line 102, in read_facade
    instance = create(mapping, facade, facade)
  File "oemof/tabular/datapackage/reading.py", line 288, in create
    instance = cls(**remap(init, attributemap, cls))
  File "oemof/tabular/facades.py", line 1407, in __init__
    self.build_solph_components()
  File "oemof/tabular/facades.py", line 1420, in build_solph_components
    nominal_value=self._nominal_value()["to_from"],
TypeError: 'NoneType' object is not subscriptable

This PR fixes that bug by introducing a conditional for the case of capacity expansion in _nominal_value.