[Backport release-3_34] [Processing] Fix regression in "Generate XYZ tiles (MBTiles)" algorithm (Fix #59986) #35985

Job	Run time
build (22.04, 5, true, OFF, ON, ON, ON, ON, OFF, OFF, ON, false)	1h 29m 0s
build (38, 6, true, ON, OFF, ON, ON, OFF, OFF, ON, OFF, false)	2h 19m 52s
clang-tidy (22.04, 5)	5m 0s
Run tests (5, ALL_BUT_PROVIDERS)	27m 12s
Run tests (5, POSTGRES)	5m 40s
Run tests (5, HANA)	5m 48s
Run tests (6, ALL_BUT_PROVIDERS)	8m 45s
Run tests (5, ORACLE, 22.04, binary-for-oracle)	4m 48s
	4h 46m 5s

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