Repository: ManimCommunity/manim
File: manim/scene/vector_space_scene.py (Python)

Code before:
        mobject in "pieces" to its corresponding .target value.
        If self.leave_ghost_vectors is True, ghosts of the original
        positions/mobjects are left on screen

        Parameters
        ----------
        pieces
            The pieces for which the movement must be shown.

        Returns
        -------
        Animation`
            The animation of the movement.
        """

        v_pieces = [piece for piece in pieces if isinstance(piece, VMobject)]
        start = VGroup(*v_pieces)
        target = VGroup(*(mob.target for mob in v_pieces))
        # don't add empty VGroups
        if v_pieces:
            if self.leave_ghost_vectors and start.submobjects:
                # start.copy() gives a VGroup of Vectors
                self.ghost_vectors.add(start.copy().fade(0.7))
                self.add(self.ghost_vectors[-1])
            return Transform(start, target, lag_ratio=0)
        return []

    def get_moving_mobject_movement(self, func: Callable[[np.ndarray], np.ndarray]):
        """
        This method returns an animation that moves a mobject
        in "self.moving_mobjects"  to its corresponding .target value.
        func is a function that determines where the .target goes.

        Parameters
        ----------

        func
            The function that determines where the .target of
            the moving mobject goes.

        Returns
        -------
        Animation
            The animation of the movement.
        """
        for m in self.moving_mobjects:
            if m.target is None:
                m.target = m.copy()
            target_point = func(m.get_center())
            m.target.move_to(target_point)
        return self.get_piece_movement(self.moving_mobjects)

Review comment (suggestion):
```suggestion
        if self.leave_ghost_vectors and start.submobjects:
            # start.copy() gives a VGroup of Vectors
            self.ghost_vectors.add(start.copy().fade(0.7))
            self.add(self.ghost_vectors[-1])
        return Transform(start, target, lag_ratio=0)
```
I don't think there is a need for this check? If `v_pieces=[]`, then `start` would have no submobjects, so the ghost vectors wouldn't get added (which I'm not sure is important, based on your PR description). Regardless, you still want this function to return an `Animation`, not an empty list.

Code after:
        mobject in "pieces" to its corresponding .target value.
        If self.leave_ghost_vectors is True, ghosts of the original
        positions/mobjects are left on screen

        Parameters
        ----------
        pieces
            The pieces for which the movement must be shown.

        Returns
        -------
        Animation
            The animation of the movement.
        """

        v_pieces = [piece for piece in pieces if isinstance(piece, VMobject)]
        start = VGroup(*v_pieces)
        target = VGroup(*(mob.target for mob in v_pieces))

        # don't add empty VGroups
        if self.leave_ghost_vectors and start.submobjects:
            # start.copy() gives a VGroup of Vectors
            self.ghost_vectors.add(start.copy().fade(0.7))
            self.add(self.ghost_vectors[-1])
        return Transform(start, target, lag_ratio=0)

    def get_moving_mobject_movement(self, func: Callable[[np.ndarray], np.ndarray]):
        """
        This method returns an animation that moves a mobject
        in "self.moving_mobjects"  to its corresponding .target value.
        func is a function that determines where the .target goes.

        Parameters
        ----------

        func
            The function that determines where the .target of
            the moving mobject goes.

        Returns
        -------
        Animation
            The animation of the movement.
        """
        for m in self.moving_mobjects:
            if m.target is None:
                m.target = m.copy()
            target_point = func(m.get_center())
            m.target.move_to(target_point)
        return self.get_piece_movement(self.moving_mobjects)