fix: android build error with React Native 0.76 (#2872)

When using React Native 0.76 the Android build fails with an error:
```
java/com/dooboolab/RNIap/PromiseUtlis.kt:40:21 Type mismatch: inferred type is String? but String was expected
```

The issue seems to be caused by the fact that in RN 0.76 the Promise
class was rewritten from Java to Kotlin and the code parameter of the
reject method is a non-nullable String now:
facebook/react-native@de73e44#diff-74cbd5a9d82e4c21dbc37dd72a92e31542e574dc6425d2454c3ce53e71e08bbeR31
but in react-native-iap it is called with a nullable string:
https://github.com/hyochan/react-native-iap/blob/12.15.6/android/src/main/java/com/dooboolab/rniap/PromiseUtlis.kt#L40.

This change updates that call to use an empty string "" for code if for
some reason it's null.

Fixes #2871

android/src/main/java/com/dooboolab/rniap/PromiseUtlis.kt

@@ -37,7 +37,7 @@ fun Promise.safeReject(
     throwable: Throwable?,
 ) {
     try {
-        this.reject(code, message, throwable)
+        this.reject(code ?: "", message, throwable)
     } catch (oce: ObjectAlreadyConsumedException) {
         Log.d(TAG, "Already consumed ${oce.message}")
     }