Refactor election vote

[JerzyStanislawski]

Refactoring nano::election::vote method according to #4108.
Main goals:

Reduce nesting of if statements
Use early returns
Simplify longer if statement.

[dsiganos]

Also, that if statement looks like an important one. And it looks like we are missing a test case for it.
Let's add a test case that checks that if statement.