Bugfix [#35686] Grease pencil to curve conversion causes NAN weights on vertices

When you convert a grease pencil stroke to a polygon curve and look at the
vertices, the first and last vertex have weight = 0, but all others have a -NaN
value. This was caused by division by zero issues when minmax_weights[0] ==
minmax_weights[1].


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57308 954f8c5b-7b00-dc11-b283-0030488c597c