Fix NullPointerException in PromiseImpl.reject by allowing nullable code

[ramanverse]

Fix NullPointerException in PromiseImpl.reject

Summary

This PR fixes a NullPointerException that occurs when Promise.reject is called with a null code from Java native modules.

The issue arises because Promise.kt and PromiseImpl.kt defined the code parameter as non-nullable String in several overloads. However, PromiseImpl's internal logic (specifically the catch-all reject method) is designed to handle null codes by defaulting to EUNSPECIFIED.

When a Java module (such as react-native-ble-plx) calls promise.reject(null, message), Kotlin's generated null-checks throw a NullPointerException before the method body is executed.

Changelog

[Android] [Fixed] - Allow nullable code in Promise.reject to prevent NPEs from Java modules

Test Plan

1. Create a native module in Java that calls promise.reject(null, "Error message").
2. Before this fix, the app crashes with java.lang.NullPointerException: Parameter specified as non-null is null: method com.facebook.react.bridge.PromiseImpl.reject, parameter code.
3. After this fix, the promise is rejected with code EUNSPECIFIED and the app does not crash.

Related Issue

Fixes #54722

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