Fix exit code 1 in CI step 'Get GIB impacted modules' on jakarta-rewrite branch

[famod]

Related to #26663

gib-impacted.log can be empty when GIB bailed out due to e.g.:

[INFO] gitflow-incremental-builder execution skipped: Changed path matches regex defined by skipIfPathMatches: .github/workflows/ci-actions-incremental.yml

This is a problem for that grep execution because it will return with exit code 1 when nothing matched.
Changing the if predicate -f to -s would be enough to fix that.

But grep will also return 1 if nothing is left, e.g. when only integration-tests/infinispan-client and/or integration-tests/kafka-avro were changed.
Therefore I replaced grep with sed (which doesn't care whether or not the respective lines are present).

[gsmet]

So that was all my fault, thanks for having a look.

[famod]

It's just too easy to miss those things!