message: fix: early return for empty statement
diff: @@ -111,13 +111,14 @@ JNIEXPORT jobject JNICALL Java_org_duckdb_DuckDBNative_duckdb_1jdbc_1prepare(JNI
  auto conn_ref = (Connection *)env->GetDirectBufferAddress(conn_ref_buf);
  if (!conn_ref) {
  env->ThrowNew(env->FindClass("java/sql/SQLException"), "Invalid connection");
  + return nullptr;
  }
  auto query = byte_array_to_string(env, query_j);
  auto statements = conn_ref->ExtractStatements(query.c_str());
  -
  if (statements.empty()) {
  env->ThrowNew(env->FindClass("java/sql/SQLException"), "No statements to execute.");
  + return nullptr;
  }
  // if there are multiple statements, we directly execute the statements besides the last one
  