StataWriter for version 117 fails on None in a string column long enough to be a Stata StrL.
The version 114 writer seems to handle columns of strings containing `None` just fine, but the 117 writer produces the `AttributeError` below.

#### Code to reproduce

```python

import pandas as pd

sample_data1 = [
    {'str1': 'string' * 30,
     'number': 0},
    {'str1': None,
     'number': 1}
]

sample_data2 = [
    {'str1': 'string' * 500,
     'number': 0},
    {'str1': None,
     'number': 1}
]

data1 = pd.DataFrame(sample_data1)
data2 = pd.DataFrame(sample_data2)

data1.to_stata('./sample1_114.dta')
data1.to_stata('./sample1_117.dta', version=117)

# Will produce the ValueError for over 244 characters.
# data2.to_stata('./sample1_114.dta')

data2.to_stata('./sample2_117.dta', version=117)

```

Error:

```python
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-23-f2347fb2dfc8> in <module>
     24 # data2.to_stata('./sample1_114.dta')
     25 
---> 26 data2.to_stata('./sample2_117.dta', version=117)

~/anaconda3/lib/python3.6/site-packages/pandas/core/frame.py in to_stata(self, fname, convert_dates, write_index, encoding, byteorder, time_stamp, data_label, variable_labels, version, convert_strl)
   1875                              write_index=write_index,
   1876                              variable_labels=variable_labels, **kwargs)
-> 1877         writer.write_file()
   1878 
   1879     def to_feather(self, fname):

~/anaconda3/lib/python3.6/site-packages/pandas/io/stata.py in write_file(self)
   2214             self._write_expansion_fields()
   2215             self._write_characteristics()
-> 2216             self._prepare_data()
   2217             self._write_data()
   2218             self._write_strls()

~/anaconda3/lib/python3.6/site-packages/pandas/io/stata.py in _prepare_data(self)
   2375                                                                self.fmtlist[i])
   2376         # 2. Convert strls
-> 2377         data = self._convert_strls(data)
   2378 
   2379         # 3. Convert bad string data to '' and pad to correct length

~/anaconda3/lib/python3.6/site-packages/pandas/io/stata.py in _convert_strls(self, data)
   2963             tab, new_data = ssw.generate_table()
   2964             data = new_data
-> 2965             self._strl_blob = ssw.generate_blob(tab)
   2966         return data
   2967 

~/anaconda3/lib/python3.6/site-packages/pandas/io/stata.py in generate_blob(self, gso_table)
   2652 
   2653             # llll
-> 2654             encoded = self._encode(strl)
   2655             bio.write(struct.pack(len_type, len(encoded) + 1))
   2656 

~/anaconda3/lib/python3.6/site-packages/pandas/io/stata.py in _encode(self, s)
   2592         """
   2593         if compat.PY3:
-> 2594             return s.encode(self._encoding)
   2595         else:
   2596             if isinstance(s, text_type):

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


#### Problem description

This seems like a fairly straightforward regression in the Stata StrL part of the 117 writer compared to how the 114 writer does some more checking on regular strings before working with them.

Note: I'm not sure if other datatypes are handled differently and may have a similar issue, but this is the one I encountered and could reproduce.

If you're wondering why my actual data is so ugly that I encountered this, I'd like to blame something else, but I wrote the code that parses the data that ends up there.


#### Expected Output

I'd expect the 117 writer to clean up these StrL columns like the 114 writer cleans up standard strings (and so does the 117 writer, because it appears to use the same codepath for standard strings and only handles StrLs differently).


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

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Darwin
OS-release: 18.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.10.0
pip: 18.1
setuptools: 40.5.0
Cython: 0.29
numpy: 1.15.4
scipy: 1.1.0
pyarrow: 0.9.0
xarray: None
IPython: 7.1.1
sphinx: 1.8.1
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.9
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.13
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: 0.1.6
pandas_gbq: None
pandas_datareader: 0.7.0

</details>