Blender Git Loki

Fix T56595: Hair Dynamics stop working after saving new file

The issue was caused by NaN valid of the average spring length being
stored in the file. This caused accumulation in the springs builder
to also deliver NaNs, which then caused solver itself to not do
anything.

Not sure why these values where never initialized prior to the
accumulation. Or even, why this runime data is stored in a DNA.

Some sanitizing is possible here, but needs to be done with care
to not disrupt Spring production.