Fix #21669: Check parents non-empty before calling reduceLeft #32589

Job	Run time
test_windows_fast	47m 22s
test	55m 30s
test_non_bootstrapped	0s
community_build_a	48m 57s
MiMa	11m 30s
test_scala2_library_tasty	0s
community_build_b	1h 17m 47s
community_build_c	1h 12m 18s
test_sbt	14m 34s
test_windows_full	0s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	8m 15s
build-chocolatey-package / build	18s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 36m 31s

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