only warn on missing json extension

[realFlowControl]

Description

Fixes #2787

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.07%. Comparing base (65395da) to head (9747b38).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2805      +/-   ##
============================================
+ Coverage     78.49%   81.07%   +2.57%     
  Complexity     2516     2516              
============================================
  Files           173      146      -27     
  Lines         18676    14647    -4029     
  Branches        975        0     -975     
============================================
- Hits          14660    11875    -2785     
+ Misses         3479     2772     -707     
+ Partials        537        0     -537     

Flag	Coverage Δ
appsec-extension	?
tracer-extension	78.19% <ø> (ø)
tracer-php	82.27% <ø> (ø)

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see 27 files with indirect coverage changes

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[bwoebi]

LGTM :-)