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Avoid an error when calling createObjectGroupsByAreas or createAreasInAreas…

Description

Avoid an error when calling createObjectGroupsByAreas or createAreasInAreas with areas argument being empty or containing an empty (sub) area.
Differential Revision: D1492
Fixes #5112
Original patch by smiley
Commandeered, edited and tested by @FeXoR
Discussed by @smiley, @elexis, @vladislavbelov and @FeXoR