fix(log): do not parse error body when response is empty in reloading config

[randmonkey]

What this PR does / why we need it:

When Kong client fails to get response from reloading declarative configuration (POST /config), do not parse the error body and directly return the error for not getting response, to produce less misleading error log (than could not unmarshal config error: unexpected end of JSON input) for the situations.

Which issue this PR fixes:

fixes #4665

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Patch coverage: 40.0% and no project coverage change.

Comparison is base (2df82cb) 67.8% compared to head (5118c3a) 67.8%.

❗ Current head 5118c3a differs from pull request most recent head 42f8b0b. Consider uploading reports for the commit 42f8b0b to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4666   +/-   ##
=====================================
  Coverage   67.8%   67.8%           
=====================================
  Files        164     164           
  Lines      19284   19289    +5     
=====================================
+ Hits       13077   13092   +15     
+ Misses      5435    5426    -9     
+ Partials     772     771    -1     

Files Changed	Coverage Δ
...al/dataplane/sendconfig/inmemory_error_handling.go	57.6% <40.0%> (-1.3%)	⬇️

... and 5 files with indirect coverage changes

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[pmalek]

I believe we best add a test case to https://github.com/Kong/kubernetes-ingress-controller/blob/56a11b9e99253fe1c0d43174173b553126cb4cc0/internal/dataplane/sendconfig/error_handling_test.go for this. WDYT?

[randmonkey]

I believe we best add a test case to https://github.com/Kong/kubernetes-ingress-controller/blob/56a11b9e99253fe1c0d43174173b553126cb4cc0/internal/dataplane/sendconfig/error_handling_test.go for this. WDYT?

Added unit test.

[pmalek]

BTW: when does POST /config return an empty body? When returning a 200? Only? Perhaps we should take the status code into account

[randmonkey]

BTW: when does POST /config return an empty body? When returning a 200? Only? Perhaps we should take the status code into account

one situation: When POST /config took too long and client timed out.