Handle empty cases better

`cat examples/secret2.txt | stack exec decrypt` doesn't return anything, probably because "habit" doesn't exist in the google dictionary. It would be nice if there could be some clever way of backtracking and ignoring certain words... or maybe just get better dictionaries

Start by reproducing `cat examples/secret2.txt | stack exec decrypt` and inspect how the decrypt command handles an empty result and dictionary lookups. Compare the behavior with `examples/secret2.txt` and the Google dictionary; a completed change would define and implement a specific fallback for missing words or an explicitly improved dictionary strategy.

Written by the indexing model from the issue text.