DuplicateBranches: don't crash on empty blocks #6837

# to view logs

Run time

Learn about OS # on GitHub Actions

Job	Run time
JDK 21 on ubuntu-latest	4m 39s
JDK 17 on ubuntu-latest	4m 47s
JDK 21 on macos-latest	4m 10s
JDK 21 on windows-latest	9m 16s
JDK 22 on ubuntu-latest	5m 8s
JDK EA on ubuntu-latest	4m 30s
Generate latest docs	0s
Publish snapshot	0s
	32m 30s