Allow the definition of None as default in process functions

[sphuber]

Fixes #1210

It was impossible to define a process function with a keyword argument
that had None as default, because the dynamically created ports always
only specified orm.Data as valid types. So the validation of the
default value would fail during the spec definition. Here we detect if
None is passed as the default in the function signature, in which case
we define the tuple of orm.Data and type(None) to be a valid type.
Note that we need to use type(None) because the port validation later
on will call isinstance(input, valid_types) which will not work if one
of the values in valid_types is simply None.

The Process._setup_inputs had to be adjusted to skip values of None
because they cannot be linked to obviously but they can now potentially
be passed as inputs to a Process.

[coveralls]

Coverage decreased (-3.5%) to 66.553% when pulling af8ae34 on sphuber:fix_1210_process_functions_allow_none_default into 838c1dd on aiidateam:provenance_redesign.