Bugfix drag and drop

[itepifanio]

Code PR

This PR fixes plotly#4374. The problem is that the Layout is initialized with no shapes and the drag and drop would trigger a relayout_data with shapes that weren't available. This PR

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• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
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  new tutorial notebook (please see the doc checklist as well).
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I actually tried to reproduce the error by writing a test programatically, but it didn't work. I wasn't able to find a way to reproduce the js events in the python code and relying on the relayout event wasn't enough to reproduce it.

def test_plotly_relayout_drag_and_drop_polygons() -> None:
    """
    Validate that the plotly_relayout method can handle drag and drop.
    """

    fig = go.FigureWidget(
        layout=go.Layout(
            showlegend=False,
            autosize=True,
            dragmode="drawrect",
            modebar={
                "add": [
                    "drawclosedpath",
                    "drawcircle",
                    "drawrect",
                    "eraseshape",
                ]
            },
        )
    )
    fig.add_shape(
        type="rect",
        x0=-0.06666666666666662,
        y0=2.4908854166666665,
        x1=2.1500000000000004,
        y1=0.3519965277777778,
    )

    new_shape = {
        "x0": -0.07964149504195264,
        "x1": 3.692616323417239,
        "y0": 3.1297222222222225,
        "y1": 1.213055555555555,
    }

    drag_and_drop_relayout_data = {
        f'shapes[0].{pos}': value for pos, value in new_shape.items()
    }

    fig.plotly_relayout(
        relayout_data=drag_and_drop_relayout_data
    )

    for pos, value in new_shape.items():
        assert fig.layout.shapes[0][pos] == value