Refactor/issue#10009 add null-check log and Convert internal-only public classes to inner classes

[sinberCS]

Which Issue(s) This PR Fixes

Fixes #10009 (comment)

Brief Description

Although the NPE (NullPointerException) issue mentioned in the issue report shouldn't occur under theoretical circumstances.
For defensive programming purposes, I added a null check at the specified location and changed the internal-only public class to an inner class.I added a null check at the specified location and changed the internal-only public class to an inner class.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.91%. Comparing base (bea086f) to head (42d986b).
⚠️ Report is 83 commits behind head on develop.

Files with missing lines	Patch %	Lines
...cketmq/broker/schedule/ScheduleMessageService.java	40.00%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #10027      +/-   ##
=============================================
+ Coverage      48.41%   48.91%   +0.50%     
- Complexity     12255    13337    +1082     
=============================================
  Files           1314     1373      +59     
  Lines          93668    99685    +6017     
  Branches       12011    12885     +874     
=============================================
+ Hits           45347    48764    +3417     
- Misses         42733    45004    +2271     
- Partials        5588     5917     +329     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.