fix(wallet) forever loading balances

Allow the balances to be nil in the BalanceDTO.

It seems in some corner-cases the balances are returned with rawBalance
as "<nil>" which can't be parsed as a valid Int256; see change cbf56fec.

"<nil>" is not a valid json value, so this is still a workaround.

Closes: #12330

src/app_service/service/wallet_account/dto/balance_dto.nim

@@ -22,7 +22,12 @@ proc getCurrencyBalance*(self: BalanceDto, currencyPrice: float64): float64 =
 
 proc toBalanceDto*(jsonObj: JsonNode): BalanceDto =
   result = BalanceDto()
-  result.rawBalance = jsonObj{"rawBalance"}.getStr.parse(Uint256)
+
+  # Expecting "<nil>" values comming from status-go when the entry is nil
+  let rawBalanceStr = jsonObj{"rawBalance"}.getStr
+  if not rawBalanceStr.contains("nil"):
+    result.rawBalance = rawBalanceStr.parse(Uint256)
+
   result.balance = jsonObj{"balance"}.getStr.parseFloat()
   discard jsonObj.getProp("address", result.address)
   discard jsonObj.getProp("chainId", result.chainId)