Cleanup: GPv3: Use Layer::is_empty()

Also adds a docstring for the `is_empty()` method.

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -420,6 +420,9 @@ class Layer : public ::GreasePencilLayer {
   const Map<FramesMapKey, GreasePencilFrame> &frames() const;
   Map<FramesMapKey, GreasePencilFrame> &frames_for_write();
 
+  /**
+   * \returns true, if the layer contains no keyframes.
+   */
   bool is_empty() const;
 
   /**

source/blender/editors/grease_pencil/intern/grease_pencil_frames.cc

@@ -348,7 +348,7 @@ bool ensure_active_keyframe(const Scene &scene,
 
   /* If auto-key is on and the drawing at the current frame starts before the current frame a new
    * keyframe needs to be inserted. */
-  const bool is_first = active_layer.sorted_keys().is_empty() ||
+  const bool is_first = active_layer.is_empty() ||
                         (active_layer.sorted_keys().first() > current_frame);
   const bool needs_new_drawing = is_first ||
                                  (*active_layer.frame_key_at(current_frame) < current_frame);