Update for Horizon API historical data changes
Due to recent changes in the Stellar Development Foundation’s Horizon API, historical data will now be limited to one year. (https://stellar.org/blog/foundation-news/sdf-s-horizon-limiting-data-to-1-year)
As a result, some claimable balance responses may return a null value for the lastModifiedTime field.
To prevent parsing issues made the lastModifiedTime field optional.