`zmax` argument not effective in `px.imshow` when `img` contains `np.nan`

Results for

import pandas as pd
import numpy as np
import plotly.express as px

df = pd.DataFrame({"foo": [1, 2, 3]})
px.imshow(df, zmax=2)

Results for

import pandas as pd
import numpy as np
import plotly.express as px

df = pd.DataFrame({"foo": [1, 2, 3, np.nan]})
px.imshow(df, zmax=2)

When df contains np.nan, zmax is not taking effect. Intesrestingly, if zmin is specified at the same time, it works.

首先运行 issue 中的两个 px.imshow 示例，并比较有无 np.nan 时 zmax 的行为。找到 px.imshow 的实现及其现有测试，然后添加覆盖，验证对于包含 np.nan 的数据，zmax=2 仍然有效；该 issue 在此回归测试通过后即完成。