`to_arrow` doesn't work correctly on Map column

[wenleix]

trafficstars

col = ta.Column([{"a": 1}, {"b1": 2, "b2": 3}])
col

0  {'a': 1}
1  {'b1': 2, 'b2': 3}
dtype: Map(string, int64), length: 2, null_count: 0

Result for to_arrow():

>>> col.to_arrow()
<pyarrow.lib.StructArray object at 0x7fcc78b432e0>
-- is_valid: all not null
-- child 0 type: int64
  [
    1,
    null
  ]
-- child 1 type: int64
  [
    null,
    2
  ]
-- child 2 type: int64
  [
    null,
    3
  ]