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

[ChiQiao]

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.