Fix crashes when connecting cyclic grease pencil curves.

Curves draw buffer updates and operations like resampling access the
evaluated lengths cache of curves. However, when the curve has only one
point the evaluated lengths will be empty. This is not a problem for
open curves, because they skip calculation of the last point, but cyclic
curves expect to have as many length entries as there are points.

This crashes in certain cases:
- Grease Pencil v3 edit mode "Toggle Cyclic" operator
- Geometry nodes resampling of cyclic, single-vertex curves

Checking for the single-point case when building draw buffers handles
these corner cases.

Fixes #117833
Fixes #117838

@@ -1011,1 +1013,4 @@
+          BLI_assert(lengths_range.is_empty());
+          continue;
+        }
         length_parameterize::accumulate_lengths(evaluated_positions.slice(evaluated_points),

@@ -605,3 +605,3 @@
       if (is_cyclic) {
         const int idx = points.size() + 1;
-        const float length = lengths[points.size() - 1];
+        const float length = (points.size() > 1 ? lengths[points.size() - 1] : 0.0f);