[Python] consistently handle conversion of all-NaN arrays across types

In ARROW-5682 (https://github.com/apache/arrow/pull/5333), next to fixing actual conversion bugs, I added the ability to convert all-NaN float arrays when converting to string type (and only with from_pandas=True). So this now works:

>>> pa.array(np.array([np.nan, np.nan], dtype=float), type=pa.string(), from_pandas=True)
<pyarrow.lib.StringArray object at 0x7f54dc9de830>
[
  null,
  null
]

However, I only added this for string type (and it already works for float and int types). If we are happy with this behaviour, we should also add it for other types.