Fix download failure for games with missing hash data

[triple-j]

It turns out that not all files will have a md5_hash. When this program tries to download those file, the error KeyError: 'md5_hash' is thrown and the rest of the downloads for that game are skipped.

This PR changes the behavior of this program so that it just skips verification of files without a MD5 hash.

Here is a collection of games that I know to be affected:

• https://adamgryu.itch.io/a-short-hike
• https://eniko.itch.io/uhd
• https://gleeson.itch.io/witchway
• https://nomnomnami.itch.io/lonely-wolf-treat (pay-what-you-want)
• https://mattmakesgames.itch.io/celeste
• https://poppyworks.itch.io/halloween-forever
• https://radicalfishgames.itch.io/crosscode (has-free-demo)