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

[asfimport]

In ARROW-5682 (#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.

Reporter: Joris Van den Bossche / @jorisvandenbossche

_{Note: This issue was originally created as ARROW-6548. Please see the migration documentation for further details.}