Prevent creation of extra else for case statements

[sharipov-ru]

PR fixes this issue: #7 by introducing if condition which checks whether else statement exists or not.

I'd also suggest to rename underscore variables _else and _whens to something like ast_else_node and ast_when_nodes, because having instructions like:

if _else

feels like a bit weird

Looking forward to hear your feedback @zeisler, I can update this PR if you approve my renaming suggestion.

[zeisler]

@sharipov-ru that looks great. I'm open for the variable name change if you want to open another pr.