Support None in integer_array

[TomAugspurger]

It mostly works

In [1]: import pandas as pd

In [2]: pd.core.arrays.integer_array([0, 1, None])
Out[2]: IntegerArray([0, 1, nan], dtype='Int64')

aside from

In [3]: pd.core.arrays.integer_array([None])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-1010251caa97> in <module>
----> 1 pd.core.arrays.integer_array([None])

~/sandbox/pandas/pandas/core/arrays/integer.py in integer_array(values, dtype, copy)
    104     TypeError if incompatible types
    105     """
--> 106     values, mask = coerce_to_array(values, dtype=dtype, copy=copy)
    107     return IntegerArray(values, mask)
    108

~/sandbox/pandas/pandas/core/arrays/integer.py in coerce_to_array(values, dtype, mask, copy)
    177                                  'mixed-integer', 'mixed-integer-float']:
    178             raise TypeError("{} cannot be converted to an IntegerDtype".format(
--> 179                 values.dtype))
    180
    181     elif not (is_integer_dtype(values) or is_float_dtype(values)):

TypeError: object cannot be converted to an IntegerDtype