empty splom in case of two dimensions

Creating a splom with two dimensions, no diagonal and only the upper or lower half results in an empty figure.

Code that reproduces the issue:

import numpy as np
import pandas as pd
import plotly.graph_objects as go

data = pd.DataFrame({"col1": np.random.rand(10), "col2": np.random.rand(10)})

fig = go.Figure(
    go.Splom(
        dimensions=[{"label": label, "values": data[label]} for label in data.columns],
        diagonal={"visible": False},
        showupperhalf=False,
        showlowerhalf=True,
    ),
)


fig.write_html("test.html")  # or fig.show()
# Gives me an empty plot. Setting `diagonal` or `showupperhalf` to `True` does give a plot

Expected result
A single scatter plot.

Comienza con la reproducción de go.Splom del issue, especialmente con las dos dimensiones, la diagonal oculta y la configuración de solo la mitad inferior. Ejecuta el script con fig.show() o fig.write_html() y sigue el comportamiento de la Figure generada. Se considera terminado cuando la splom bidimensional de solo la mitad inferior renderiza su único scatter plot en lugar de una Figure vacía.