Fixed crash on trying send bytes after unpair

[ILYA-2606]

**Description:** The `sendBytes` method uses force unwrap of `rxCharacteristic`. This leads to a crash if you try to send bytes after disconnecting from the glasses

Reproduction steps:

1. Connect to the ActiveLook glasses
2. Unpair the glasses
3. Try to send bytes (For example, read the settings)

Expected Result:
If rxCharacteristic is nil, then the sendBytes method should not attempt to send bytes

Actual Result:
Unhandled exception because rxCharacteristic is nil