!19413 merge fix-leakobj-empty-check into master

fix: 防止GetDumpResult将空字符串赋值给dumpResult

Created-by: denganliang
Commit-by: denganliang
Merged-by: openharmony_ci
Description: ## 修改说明

- 防止 GetDumpResult 将 malloc 分配但内容为空字符串的 buf 赋值给 dumpResult
- 根因：当 OHOS_GetMemLeakString malloc 分配了缓冲区但内容为空字符串时，原代码只判断 buf!=nullptr 就赋值
- 增加 *buf!=空字符 判断，与 buf!=nullptr 一起构成双重防御

**IssueNo**:
https://gitcode.com/openharmony/hiviewdfx_hidumper/issues/586

## 关联PR

- openharmony/hiviewdfx_hidumper!767

## 验证

- 编译通过
- 与 hidumper PR 配合验证

See merge request: openharmony/ability_ability_runtime!19413

frameworks/native/appkit/app/dump_runtime_helper.cpp

@@ -417,7 +417,7 @@ bool DumpRuntimeHelper::GetDumpResult(std::string &dumpResult)
     bool ret = func(TYPE_TXT, &buf, MEM_LEAK_MAX_SIZE);
     if (!ret) {
         TAG_LOGE(AAFwkTag::APPKIT, "TYPE_TXT Error");
-    } else if (buf != nullptr) {
+    } else if (buf != nullptr && *buf != '\0') {
         dumpResult = buf;
         TAG_LOGI(AAFwkTag::APPKIT, "TYPE_TXT finish, result:%{public}s", dumpResult.c_str());
     }