Do you mean to say, how " " + " " is still an empty string?

It's because it's two (or more) whitespaces together, the result of above just being " " (now two whitespaces).

Since the task is to remove spaces to make the result a proper hashtag, it's probably safe to assume we should remove all spaces no matter how many might be in between two words.

yes, I'm aware that my code here and for many other kata isn't written according to all best practices and common standarts as it is not intended to be a production code that should be changed and supported later. And you're right, it is completely ok to advice a form you see as more readable. I'm also new to Ruby.
Thanks for the support

Please use spoiler flag next time.