ENH: Add na_value to DataFrame.to_numpy

[TomAugspurger]

Is your feature request related to a problem?

Control the NA value used when converting a DataFrame to an ndarray.

Describe the solution you'd like

Add an na_value argument to DataFrame.to_numpy(). Has the same meaning as na_value in Series.to_numpy, except it applies to all the columns.

Additional context

In [8]: df = pd.DataFrame({"A": pd.array([1, None])})

In [9]: df.to_numpy(na_value=np.nan)

This has come up in a few places (cc @jorisvandenbossche @dsaxton)

[dsaxton]

Thanks @TomAugspurger. #33809 took the approach of using DataFrame.astype(float).to_numpy() to coerce NA to np.nan which should likely be fixed once this becomes available.

pandas/pandas/core/frame.py

Line 7873 in 8aa7072

mat = numeric_df.astype(float, copy=False).to_numpy()