If nothing else, Cortana liked a challenge.
Does it mean
Maybe, challenge was Cartana's only hobby.
"If nothing else" normally means something like, "for no other reason". For example
Why don't you come with me to the party? If nothing else, Margaret will be there, and you like talking to her.
The speaker here is saying that, even if you have no other good reason to come to the party, at least you can enjoy talking to Margaret.
In your book, the use of this phrase is a little less clear, but also mostly meaningless. "If nothing else" is just an easy way to imply that, even if Cortana had no other reason to try and crack the code (and it seems she does have a good reason), she would try because it was a "good challenge".
Otherwise, it implies nothing about Cortana's hobbies or preferences.