refactor: replace null value with empty string

[frank-zsy]

Signed-off-by: Frankzhaopku syzhao1988@126.com

Purpose | 本PR解决的问题

Changes | 本PR的更改

• 将 null 值改为空字符串

Checklist | 检查

• Are tests written for added code? | 新增代码是否添加测试用例

• Yes
• No because:

• Did you introduce any new dependencies? | 如果添加新的依赖，请列出依赖并说明添加该依赖的原因

• No
• Yes
  • Dependency:
  • Reason:

[codecov-io]

Codecov Report

Merging #64 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   26.19%   26.07%   -0.12%     
==========================================
  Files          18       18              
  Lines         439      441       +2     
  Branches       24       25       +1     
==========================================
  Hits          115      115              
- Misses        324      326       +2

Impacted Files	Coverage Δ
app/service/data_format.ts	30.55% <0%> (-0.58%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 489cf59...7118895. Read the comment docs.