Fix: OffsetIndices assert with invalid span

In some cases (when there are no faces for example, the offsets span
can have a size of 1 but no data). That's technically invalid and might
need to be addressed more later on, but for now, just fix the assert.

source/blender/blenlib/BLI_offset_indices.hh

@@ -32,7 +32,7 @@ template<typename T> class OffsetIndices {
   OffsetIndices() = default;
   OffsetIndices(const Span<T> offsets) : offsets_(offsets)
   {
-    BLI_assert(std::is_sorted(offsets_.begin(), offsets_.end()));
+    BLI_assert(offsets_.size() < 2 || std::is_sorted(offsets_.begin(), offsets_.end()));
   }
 
   /** Return the total number of elements in the referenced arrays. */