oneOf fails to match when block txs is empty

[lightclient]

In cases where transactions is an empty array [], the oneOf keyword matches both array items since it can't differentiate. This causes schema validation to fail, because oneOf must match only one of the item subschemas.

Two possible fixes:

1. Change oneOf to anyOf. The subschemas deviate enough that the only time both can match is in the case of an empty array.
2. Keep oneOf but set minItems of one of the item schemas to 1. This way, the empty array would only match one item.

I went with option 1) because it seems more correct. Under 2), the schema would make the distinction that the empty array is definitely one item type or the other, when in reality there isn't enough information for it to make that conclusion.