BUG: combine_first fails when one series is timezone-aware and the other is emtpy.
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [ ] \(optional) I have confirmed this bug exists on the master branch of pandas.

#### Code Sample, a copy-pastable example
```python 
import pandas as pd
import numpy as np
from datetime import datetime

>>> time_index = pd.date_range(datetime(2021, 1, 1, 1), datetime(2021, 1, 1, 10), freq='H', tz='Europe/Oslo')
>>> s1 = pd.Series(np.random.random(10), index=time_index)
>>> s2 = pd.Series()
>>> s1.combine_first(s2)
```
 **Traceback**

<details>

```python
AttributeError                            Traceback (most recent call last)
<ipython-input-14-9af79f353bf5> in <module>
----> 1 s1.combine_first(pd.Series())

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/series.py in combine_first(self, other)
   2983             other = to_datetime(other)
   2984
-> 2985         return this.where(notna(this), other)
   2986
   2987     def update(self, other) -> None:

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/generic.py in where(self, cond, other, inplace, axis, level, errors, try_cast)
   9285         other = com.apply_if_callable(other, self)
   9286         return self._where(
-> 9287             cond, other, inplace, axis, level, errors=errors, try_cast=try_cast
   9288         )
   9289

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/generic.py in _where(self, cond, other, inplace, axis, level, errors, try_cast)
   9019         cond = com.apply_if_callable(cond, self)
   9020         if isinstance(cond, NDFrame):
-> 9021             cond, _ = cond.align(self, join="right", broadcast_axis=1)
   9022         else:
   9023             if not hasattr(cond, "shape"):

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/series.py in align(self, other, join, axis, level, copy, fill_value, method, limit, fill_axis, broadcast_axis)
   4228             limit=limit,
   4229             fill_axis=fill_axis,
-> 4230             broadcast_axis=broadcast_axis,
   4231         )
   4232

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/generic.py in align(self, other, join, axis, level, copy, fill_value, method, limit, fill_axis, broadcast_axis)
   8832                 method=method,
   8833                 limit=limit,
-> 8834                 fill_axis=fill_axis,
   8835             )
   8836         else:  # pragma: no cover

~/anaconda3/envs/powerml_dev/lib/python3.7/site-packages/pandas/core/generic.py in _align_series(self, other, join, axis, level, copy, fill_value, method, limit, fill_axis)
   8985         if is_series or (not is_series and axis == 0):
   8986             if is_datetime64tz_dtype(left.index.dtype):
-> 8987                 if left.index.tz != right.index.tz:
   8988                     if join_index is not None:
   8989                         # GH#33671 ensure we don't change the index on

AttributeError: 'Index' object has no attribute 'tz'
```

</details>

#### Problem description
Combine fails when one series has a timezone-aware datetime index and the other is empty. This does not happen if s1 has naive datetimes.

#### Expected output
The output should be equal to s1.

#### Output of ```python pd.show_versions()```
<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-142-generic
Version          : #146-Ubuntu SMP Tue Apr 13 01:11:19 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.19.0
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.2.0.post20200714
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.1
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
