`InputPort.default` raises a `RuntimeError` if no default was defined

[sphuber]

plumpy/src/plumpy/ports.py

Line 271 in ff5770f

raise RuntimeError('No default')

This is positively weird design. Not sure what the original reasoning was but of all the ways of communicating to the caller that no default was set, this has got to be one of the least expected ones. We should probably just have it defined the UNSPECIFIED tuple.