CAMEL-23451: skip non-module files in incremental build instead of aborting

[gnodet]

CAMEL-23451

Regression from #22247: when the CI rationalization removed the buildAll mode, the
projectRoot == "." case was changed from "build everything" to exit 0 (skip all tests).
This means files in directories without a pom.xml (e.g. proposals/*.adoc, README.md)
cause findProjectRoot to return ".", and the script aborts all test execution — even
when the PR also contains real code changes.

For example, PR #23002 ran no tests because
it modified proposals/tracing.adoc.

Fix: Skip non-module files with continue instead of aborting. Root-level POM changes
are already handled separately by the grep and Scalpel dependency analysis in Step 2.

Test plan

• PR that touches only proposals/*.adoc → no modules tested (correct, doc-only)
• PR that touches proposals/*.adoc + component code → component tests run (was broken, now fixed)
• PR that touches only component code → unchanged behavior

[github-actions]

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[squakez]

Once merged, please let's close: CAMEL-23451