[Backport release-3_40] [Processing] Fix regression in "Generate XYZ tiles (MBTiles)" algorithm (Fix #59986) #36030

Job	Run time
build (22.04, 5, true, OFF, ON, ON, ON, ON, OFF, OFF, OFF, OFF, ON, false)	1h 44m 23s
build (39, 6, true, ON, OFF, ON, ON, OFF, OFF, ON, ON, ON, OFF, false)	30m 27s
clang-tidy (22.04, 5)	7m 35s
Run tests (5, ALL_BUT_PROVIDERS)	30m 4s
Run tests (5, POSTGRES)	6m 26s
Run tests (5, HANA)	5m 16s
Run tests (6, ALL_BUT_PROVIDERS)	38m 38s
Run tests (5, ORACLE, 22.04, binary-for-oracle)	8m 0s
	3h 50m 49s

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