Fix: Prevent Android crash when BLE disconnect error has null error code

Summary

This PR fixes a fatal Android crash that occurs when Promise.reject() is called with a null error code during BLE disconnect flows.

On Android, RxAndroidBle emits a BleDisconnectedException for both expected and unexpected disconnects. In some cases, the propagated error does not contain a non-null error code, which violates React Native's Promise.reject contract and results in a NullPointerException.

This change ensures a safe fallback error code is always provided.

Problem

During normal BLE lifecycle events (navigation cleanup, explicit disconnect, device power loss), the app may crash with:

java.lang.NullPointerException:
Parameter specified as non-null is null:
method com.facebook.react.bridge.PromiseImpl.reject, parameter code

This crash occurs even when the disconnect is expected and properly handled in JavaScript.

Root Cause

SafePromise.reject(...) forwards a nullable code value into Promise.reject(code, ...).

React Native requires code to be non-null, which causes a runtime crash.

Solution

Provide a defensive fallback error code when code is null before calling Promise.reject.

Example:

String safeCode = code != null ? code : "UNKNOWN_ERROR";
promise.reject(safeCode, message, throwable);

Impact

• Prevents Android crashes during normal BLE disconnect flows
• No behavior change for JS consumers
• Improves stability in real-world BLE lifecycle scenarios