Fixed purchaseUris by making it optional.

[jonahgabriel]

There are a number of sets that contain cards with missing keys. This causes a decoding error when trying to call set.getCards() which returns an empty array. This PR fixes the issue by making purchaseUris optional.

▿ Result<CardList> ▿ failure : DecodingError ▿ keyNotFound : 2 elements - .0 : CodingKeys(stringValue: "purchaseUris", intValue: nil) ▿ .1 : Context ▿ codingPath : 2 elements - 0 : CodingKeys(stringValue: "data", intValue: nil) ▿ 1 : _JSONKey(stringValue: "Index 15", intValue: 15) - stringValue : "Index 15" ▿ intValue : Optional<Int> - some : 15 - debugDescription : "No value associated with key CodingKeys(stringValue: \"purchaseUris\", intValue: nil) (\"purchaseUris\"), converted to purchase_uris." - underlyingError : nil

Affected sets are: ddf, me3, me1, psus, 6ed, 5ed, ren, 4bb, 4ed, sum, leg, 3ed, fbb, arn, cei, ced, 2ed, leb,lea