[bug 933786] Fix strip error

We get this weird error on input-stage (and only on stage) where someone
is using the Input API and sending in nonsensical POST data and it
somehow manages to result in attrs['device'] being None which causes an
error.

I spent some time looking into it and I can't figure out how it ends up
as a None. I don't know what the actual input that's causing the error
is.

Regardless, this alleviates that by making sure it's a unicode but it's
goofipants.

r?