[STUMP] allow custom types to return nil

[Eric Abrahamsen]

I've been working on another patch to the manual, documenting commands
and custom types. In the course of writing an example of a custom type,
I discovered that you can't have a type return a nil value: it aborts
the interactive call. This is mentioned in a FIXME in
`call-interactively'.

For more complicated custom types I don't see why returning a nil value
should make the command blow up altogether. I've attached a patch which
allows nil return values -- I doubt it will cause any problems, but I'm
also happy to just run it locally for a few weeks first and see if
anything happens.

E

0001-Allow-types-to-return-nil-values.patch
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