[fix][admin] Map empty content to null

[nodece]

Motivation

When using asyncResponse.resume(null) to write data to the admin, we will get an empty content, not null.

Please see the Jersey implementation: https://github.com/eclipse-ee4j/jersey/blob/2.34/core-server/src/main/java/org/glassfish/jersey/server/ServerRuntime.java#L603-L610

Modifications

In the asyncGetRequest method, when the content is empty, use null as the completion value.

Documentation

• [ ] doc
• [ ] doc-required
• [x] doc-not-needed
• [ ] doc-complete

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 57.14286% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 71.85%. Comparing base (a702e5a) to head (5639be7). Report is 56 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #22008      +/-   ##
============================================
- Coverage     73.48%   71.85%   -1.63%     
+ Complexity    32465    32281     -184     
============================================
  Files          1863     1877      +14     
  Lines        138812   147777    +8965     
  Branches      15215    17345    +2130     
============================================
+ Hits         102003   106184    +4181     
- Misses        28921    33161    +4240     
- Partials       7888     8432     +544     

Flag	Coverage Δ
inttests	27.18% <0.00%> (?)
systests	24.21% <21.42%> (+0.30%)	:arrow_up:
unittests	70.99% <57.14%> (-1.89%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...che/pulsar/client/admin/internal/BaseResource.java	76.27% <57.14%> (-2.05%)	:arrow_down:

... and 182 files with indirect coverage changes