Concat of dataframe with tz-aware datetime column against dataframe without, fails
```python
a = pd.DataFrame([[1, 2]], dtype='datetime64[ns, UTC]')
b = pd.DataFrame([[3]], dtype='datetime64[ns, UTC]')
pd.concat([a, b])
```

Fails:
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-42-457226d62f27> in <module>()
      1 a = pd.DataFrame([[1, 2]], dtype='datetime64[ns, UTC]')
      2 b = pd.DataFrame([[3]], dtype='datetime64[ns, UTC]')
----> 3 pd.concat([a, b])

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/reshape/concat.py in concat(objs, axis, join, join_axes, ignore_index, keys, levels, names, verify_integrity, sort, copy)
    224                        verify_integrity=verify_integrity,
    225                        copy=copy, sort=sort)
--> 226     return op.get_result()
    227 
    228 

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/reshape/concat.py in get_result(self)
    421             new_data = concatenate_block_managers(
    422                 mgrs_indexers, self.new_axes, concat_axis=self.axis,
--> 423                 copy=self.copy)
    424             if not self.copy:
    425                 new_data._consolidate_inplace()

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/internals.py in concatenate_block_managers(mgrs_indexers, axes, concat_axis, copy)
   5419         else:
   5420             b = make_block(
-> 5421                 concatenate_join_units(join_units, concat_axis, copy=copy),
   5422                 placement=placement)
   5423         blocks.append(b)

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/internals.py in concatenate_join_units(join_units, concat_axis, copy)
   5563     to_concat = [ju.get_reindexed_values(empty_dtype=empty_dtype,
   5564                                          upcasted_na=upcasted_na)
-> 5565                  for ju in join_units]
   5566 
   5567     if len(to_concat) == 1:

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/internals.py in <listcomp>(.0)
   5563     to_concat = [ju.get_reindexed_values(empty_dtype=empty_dtype,
   5564                                          upcasted_na=upcasted_na)
-> 5565                  for ju in join_units]
   5566 
   5567     if len(to_concat) == 1:

~/.pyenv/versions/3.6.2/envs/general/lib/python3.6/site-packages/pandas/core/internals.py in get_reindexed_values(self, empty_dtype, upcasted_na)
   5849 
   5850             if not self.indexers:
-> 5851                 if not self.block._can_consolidate:
   5852                     # preserve these for validation in _concat_compat
   5853                     return self.block.values

AttributeError: 'NoneType' object has no attribute '_can_consolidate'
```

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-32-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.0.6
pip: 9.0.1
setuptools: 39.1.0
Cython: 0.28.3
numpy: 1.15.1
scipy: 1.1.0
pyarrow: 0.9.0
xarray: 0.10.0
IPython: 6.2.1
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: 1.1.10
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None
</details>
