Blender Git Loki

Fix T74648: Do not relax with 0 neighbors or no vertex normal

The mesh provided in the report has 0 area faces and overlapping
vertices, causing the relax code to fail when calculating the plane to
constraint the vertex movement. Now it works fine both in the brush and
in the mesh filter.

Reviewed By: jbakker

Maniphest Tasks: T74648

Differential Revision: https://developer.blender.org/D7109