Fix index error

[scottnuma]

Make _format_collection compatible with empty collections

• When loading up py-slippi and loading a .slp, I hit an IndexError when trying to view game.frames
• Digging into what was happening with pdb, I found that _format_collection was being called where coll was an empty tuple
• After adding this change, was able to successfully view game.frames

[hohav]

Thanks for the fix!

Nitpick: it's idiomatic to use the fact that evaluating a list in a boolean context will return True iff the list is non-empty. So could you change len(elements) > 0 to just elements?

[scottnuma]

patched it up (y)

[hohav]

Thanks!