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Fix T83856: Sculpt: anchored brushes with spherical falloff ignore topology automasking

Anchored brushes with spherical falloff start off with zero radius, thus
we have no pbvh nodes on the first brush step. This would prevent
initializing the automasking cache [which only happens on the first brush
step].

Maniphest Tasks: T83856

Differential Revision: https://developer.blender.org/D9873