handle missing account properties for GerritChangeSource

* create a separate function to convert account properties
* make use of the function in all cases where the situation might take place
* update test cases
Well, I'd like to know more of the real problems you are trying to tackle with this best practice.
This is an old habbit that is also in the core of the status string system, that has been confusing (at least my team) a lot. (see #1238)

If you dont care, I would prefer to stick the best practice with the easy to understand string manipulation.