Fix: Small fixes for NURBS evaluation

Clear the nurbs basis cache dirty flag when its evaluation finishes.
Remove an incorrect assert that the evaluated size couldn't be zero.
It can, when `check_valid_size_and_order` returns false.

source/blender/blenkernel/intern/curve_nurbs.cc

@@ -154,7 +154,6 @@ void calculate_basis_cache(const int points_num,
                            BasisCache &basis_cache)
 {
   BLI_assert(points_num > 0);
-  BLI_assert(evaluated_size > 0);
 
   const int8_t degree = order - 1;
 

source/blender/blenkernel/intern/curves_geometry.cc

@@ -578,6 +578,8 @@ void CurvesGeometry::ensure_nurbs_basis_cache() const
       }
     });
   });
+
+  this->runtime->nurbs_basis_cache_dirty = false;
 }
 
 Span<float3> CurvesGeometry::evaluated_positions() const