[Python] DuckDB -> Pandas | NULL -> str becomes None instead of 'NaN'

[Tishj]

This PR fixes #8652 and a bunch of others that raise the same issue.

Changes made:
Separated the giant file array_wrapper.cpp and array_wrapper.hpp into:
raw_array_wrapper, array_wrapper and numpy_result_conversion

Bundled all parameters used in the internal functions of ArrayWrapper::Append into NumpyAppendData.

changed NUMPY_T NullValue<class NUMPY_T>() to:
NUMPY_T NullValue<class NUMPY_T, bool PANDAS>(bool &set_mask)

This allows the NullValue function to be in charge of whether the mask gets set, and change behavior based on if PANDAS is set or not.

The only NullValue that has been changed for now is StringConvert, it's now:

	template <class NUMPY_T, bool PANDAS>
	static NUMPY_T NullValue(bool &set_mask) {
		if (PANDAS) {
			set_mask = false;
			return Py_None;
		}
		set_mask = true;
		return nullptr;
	}

The NumpyResultConversion has a new constructor parameter, bool pandas = false.
This gets forwarded to the ArrayWrapper created on construction.

[Mytherin]

Thanks! LGTM