Why is None an illegal revision property value?

[Ian Clatworthy]

I'm in the process of trying to make fast-export/import round-trip
bzr.dev and I'm running into some issues around properties. In
particular, there are 67 cases where the 'bugs' property is empty and/or
None which is currently being treated as None by the exporter. I'm
fixing that so that an empty string stays an empty string but it raises
a question in my mind ...

The import is falling over because None is an illegal property value.
Why do we have that rule, i.e. why don't we allow None as a legal
property value? Why is an empty string legal but None not?

If there's no good reason, any objections to a small patch to change it?
If nothing else, it will help roundtripping from/to other systems that
may allow that value.

Ian C.