Fix #122503: GPv3: Always Initialize vertex_color_ when drawing

Lack of initialization of `vertex_color_` will cause garbled color to
show up in later interpolated points while drawing in material mode with
curves already have `vertex_color` attribute. Now fixed.

Pull Request: https://projects.blender.org/blender/blender/pulls/122528

source/blender/editors/sculpt_paint/grease_pencil_paint.cc

@@ -185,6 +185,9 @@ struct PaintOperationExecutor {
                         std::make_optional(color_base) :
                         std::nullopt;
     }
+    else {
+      vertex_color_ = std::make_optional(ColorGeometry4f(0.0f, 0.0f, 0.0f, 0.0f));
+    }
     softness_ = 1.0f - settings_->hardness;
 
     BLI_assert(grease_pencil->has_active_layer());