[4.6.x] fix: ingore Unknown sub user during introspect

[mergify]

This is an automatic copy of pull request #5118 done by Mergify.

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in some cases sub may not be the client_id but still not represent a user (ex with extension grant)

fixes AM-4340

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Cherry-pick of 5130d22 has failed:

On branch mergify/bp/4.6.x/pr-5118
Your branch is up to date with 'origin/4.6.x'.

You are currently cherry-picking commit 5130d22dbf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/introspection/impl/IntrospectionServiceImpl.java
	both modified:   gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/test/java/io/gravitee/am/gateway/handler/oauth2/service/introspection/IntrospectionServiceTest.java

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

Cherry-pick of 5130d22 has failed:

On branch mergify/bp/4.6.x/pr-5118
Your branch is up to date with 'origin/4.6.x'.

You are currently cherry-picking commit 5130d22dbf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/main/java/io/gravitee/am/gateway/handler/oauth2/service/introspection/impl/IntrospectionServiceImpl.java
	both modified:   gravitee-am-gateway/gravitee-am-gateway-handler/gravitee-am-gateway-handler-oidc/src/test/java/io/gravitee/am/gateway/handler/oauth2/service/introspection/IntrospectionServiceTest.java

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[leleueri]

4.6.x not impacted by the bug (we already fallback to a default value)