ios: don't decode product info if empty

Will generate a needless decoding error. The product value is empty if
not read yet or if the device is not in bootloader/firmware state.

Author: benma

frontends/ios/BitBoxApp/BitBoxApp/Bluetooth.swift

@@ -297,6 +297,10 @@ class BluetoothManager: NSObject, ObservableObject, CBCentralManagerDelegate, CB
               let value = pProduct.value else {
             return nil
         }
+        
+        if value.isEmpty {
+            return nil
+        }
 
         do {
             let decoder = JSONDecoder()