optimize: add null value check for MAC address

[bageyang]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

add null value check for MAC address

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 50.85%. Comparing base (d318409) to head (3d1d104).
Report is 8 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #6458      +/-   ##
============================================
+ Coverage     50.81%   50.85%   +0.03%     
  Complexity     5285     5285              
============================================
  Files           942      942              
  Lines         33270    33270              
  Branches       4030     4030              
============================================
+ Hits          16907    16919      +12     
+ Misses        14698    14685      -13     
- Partials       1665     1666       +1     

Files	Coverage Δ
...in/java/org/apache/seata/common/util/IdWorker.java	83.33% <50.00%> (ø)

... and 2 files with indirect coverage changes

[funky-eyes]

请登记pr和作者信息至https://github.com/apache/incubator-seata/tree/2.x/changes 中的中英两版2.x.md文件
Please register PR and author information to the Chinese and English 2.x.md in the https://github.com/apache/incubator-seata/tree/2.x/changes

[slievrly]

LGTM