[Backport release-3_34] Prevent calling findLayers when there is no legend->rootGroup (fix #57595) #33769

Job	Run time
build (22.04, 5, true, OFF, ON, ON, ON, ON, OFF, OFF, ON, false)	1h 28m 55s
build (38, 6, true, ON, OFF, ON, ON, OFF, OFF, ON, OFF, false)	11m 9s
clang-tidy (22.04, 5)	3m 55s
Run tests (5, ALL_BUT_PROVIDERS)	28m 52s
Run tests (5, POSTGRES)	5m 4s
Run tests (5, HANA)	5m 5s
Run tests (6, ALL_BUT_PROVIDERS)	8m 54s
Run tests (5, ORACLE, 22.04, binary-for-oracle)	5m 43s
	2h 37m 37s

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