Reconsider returning the configured undefined for else-less ifs

[adrian-zon]

(I would have just commented on #710 but it's locked)

The current behavior of always returning stock Undefined, regardless of the configured undefined class, is highly surprising. I understand the wish to be able to write {{ 'foo' if false }}, but we use a lot of if clauses for setting variables or function parameters, and I would not have expected to find Undefined instances when having undefined set to another class.

I suggest to either return a different value depending on the context (this is probably difficult or at least annoying to implement), or always return a defined value like None.