Fix a false-positive context leak error when using RetryingClient

[ikhoon]

Motivation:

Please refer to #6420 for a detail explanation of the problem.

Modifications:

• When ServiceRequestContext is pushed, no exception is thrown if the root() of the current ClientRequestContext is null, since it is not considered a context leak.

Result:

• RetryingClient no longer raises a false-positive context leak error.
• Closes False-positive RequestContext leak warning when using RetryingClient #6420

[github-actions]

🔍 Build Scan® (commit: f59a789)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/nu5dtxphyywu2
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/et3aa7th226mu
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/ahlv6wqlx67ii
build-ubicloud-standard-16-jdk-17-min-java-11	❌ (failure)	https://ge.armeria.dev/s/lts5cvbnclgje
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/yv4vrnccqunoi
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/ejapskzom6djq
build-macos-latest-jdk-21	❌ (failure)	https://ge.armeria.dev/s/2ghvjsbskm6mo

[minwoox]

👍 👍

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.10%. Comparing base (8150425) to head (f59a789).
⚠️ Report is 215 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6479      +/-   ##
============================================
- Coverage     74.46%   74.10%   -0.36%     
- Complexity    22234    22999     +765     
============================================
  Files          1963     2062      +99     
  Lines         82437    86130    +3693     
  Branches      10764    11311     +547     
============================================
+ Hits          61385    63826    +2441     
- Misses        15918    16888     +970     
- Partials       5134     5416     +282     

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