examples: fix android example cannot be generated continuously (ggml-org#8621)

When generation ends `completion_loop()` should return a NULL, not the empty string

Author: devojony

examples/llama.android/llama/src/main/cpp/llama-android.cpp

@@ -409,7 +409,7 @@ Java_android_llama_cpp_LLamaAndroid_completion_1loop(
 
     const auto n_cur = env->CallIntMethod(intvar_ncur, la_int_var_value);
     if (llama_token_is_eog(model, new_token_id) || n_cur == n_len) {
-        return env->NewStringUTF("");
+        return nullptr;
     }
 
     auto new_token_chars = llama_token_to_piece(context, new_token_id);