Handle pub variable with no value

[ascholerChemeketa]

If a pub variable results in an empty string the get_publisher_varaible function chokes.

This provides None for missing values. Defaulting to "" might require less code for handling but also be more likely to result in sneaky errors based on assuming a non-empty string came back.

[rbeezer]

Nice catch! Had not thought that through.

I've spent a long time on this one. Mostly because there is an existing test on the return value being an empty string.

I agree with everything you have said. But I think I want to keep this function true to what it says it does - the publisher variable may indeed be an empty string, and in XSL-land that is frequently used for the same sorts of things None is. (And this is not simply to avoid changes to behavior or employment.) So I have addressed the bug, by returning an empty string.

I've kept your name on this - I hope that is OK.