fix: handle null values when deserializing byte arrays

[neilcampbell]

The indexer API supports retrieving deleted applications by suppling the include-all=true param when calling /v2/applications/{applicationId}, however when fetching a deleted application via the algosdk using indexer.lookupApplications(applicationId).includeAll(true).do() it throws an error "Invalid byte array: (object) null".

This error is due to the indexer API returning a null value for the approval-program and clear-state-program, however those values are deserialized using the ByteArraySchema which expects a base64 encoded bytes value. This can be observed in this application https://mainnet-idx.algonode.cloud/v2/applications/2365972864?include-all=true

This PR updates the ByteArraySchema JSON deserializer to handle null or undefined values and maps them to the default value (empty byte array).