Handling None -> Missing conversions in matrices

Hi,

I have noticed that trying to convert a numpy array containing `None` to a `Matrix{Union{Missing, Float64}}` does not work smoothly. Indeed, it gives the following error: `TypeError('must be real number, not NoneType')`.

Have you ever encountered this issue before? Are there any working solutions?

Reproduce the reported conversion in PyCall.jl using a NumPy array containing None and a Julia Matrix{Union{Missing, Float64}}. Trace the conversion path that raises TypeError('must be real number, not NoneType'); done should be a defined, tested behavior for this input or a documented working solution.

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