[fix] [admin] Fix lookup get a null result if uses proxy

[poorbarcode]

Motivation

If the user deploys a cluster with proxy, and network topology like this: Client -> Proxy(with TLS) -> Broker(without TLS)

When the user calls pulsar-admin topics lookup <topic name>, the user will get a null response.

But the exact value that the broker responded to is below:

{
	"brokerUrl": "pulsar://broker-1.com:6650",
	"httpUrl": "http://broker-1.com:8080",
	"brokerUrlSsl": ""
}

It is caused by the pulsar-admin client's issue: https://github.com/apache/pulsar/blob/master/pulsar-client-admin/src/main/java/org/apache/pulsar/client/admin/internal/LookupImpl.java#L60

if (client.useTls) {
  return response.brokerUrlSsl; // even if it is null 
}

Modifications

• fix the issue
• the test is hard to write, so I skipped to write a test

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: x

[codecov-commenter]

Codecov Report

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

Project coverage is 74.31%. Comparing base (bbc6224) to head (e6df03d).
Report is 710 commits behind head on master.

Files with missing lines	Patch %	Lines
...pache/pulsar/client/admin/internal/LookupImpl.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #23556      +/-   ##
============================================
+ Coverage     73.57%   74.31%   +0.74%     
- Complexity    32624    34424    +1800     
============================================
  Files          1877     1943      +66     
  Lines        139502   147051    +7549     
  Branches      15299    16209     +910     
============================================
+ Hits         102638   109288    +6650     
- Misses        28908    29330     +422     
- Partials       7956     8433     +477     

Flag	Coverage Δ
inttests	27.37% <0.00%> (+2.79%)	⬆️
systests	24.31% <0.00%> (-0.01%)	⬇️
unittests	73.70% <50.00%> (+0.85%)	⬆️

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

Files with missing lines	Coverage Δ
...pache/pulsar/client/admin/internal/LookupImpl.java	73.91% <50.00%> (-1.65%)	⬇️

... and 652 files with indirect coverage changes