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.

从 issue 中的 go.Splom 复现开始，重点关注两个维度、隐藏的对角线以及仅显示下半部分的设置。使用 fig.show() 或 fig.write_html() 运行脚本，并跟踪生成的 Figure 行为。完成的标准是：二维、仅显示下半部分的 splom 能够渲染其单个 scatter plot，而不是空的 Figure。