Iterating on lines of S3 empty files generates exception
My code looks like:
```python
    def process_file(filename, f):
        log.info("Indexing file %s", filename)
        for line in f:
(...)
    s3 = boto3.resource('s3')
    for o in s3.Bucket(bucket).objects.filter(Prefix=path):
        with closing(o.get()['Body']) as f:
            for line in process_file('s3://' + o.bucket_name + "/" + o.key, f.iter_lines()):
                yield line
```

With botocore>=1.12.68 (including 1.12.71) this gives:
```
  File "main.py", line 68, in gen_bulk_data
    for line in process_file('s3://' + o.bucket_name + "/" + o.key, f.iter_lines()):
  File "main.py", line 57, in process_file
    for line in f:
  File "/usr/local/lib/python3.5/site-packages/botocore/response.py", line 117, in iter_lines
    yield pending.splitlines()[0]
IndexError: list index out of range
```
It works with botocore<1.12.68
diff --git a/botocore/response.py b/botocore/response.py
index 13a15f9fd..93c7937aa 100644
--- a/botocore/response.py
+++ b/botocore/response.py
@@ -114,7 +114,8 @@ class StreamingBody(object):
             for line in lines[:-1]:
                 yield line.splitlines()[0]
             pending = lines[-1]
-        yield pending.splitlines()[0]
+        if pending:
+            yield pending.splitlines()[0]
 
     def iter_chunks(self, chunk_size=_DEFAULT_CHUNK_SIZE):
         """Return an iterator to yield chunks of chunk_size bytes from the raw
diff --git a/tests/unit/test_response.py b/tests/unit/test_response.py
index d1e1fd8ce..6ad43e537 100644
--- a/tests/unit/test_response.py
+++ b/tests/unit/test_response.py
@@ -167,6 +167,12 @@ class TestStreamWrapper(unittest.TestCase):
                 [b'1234567890', b'1234567890', b'12345'],
             )
 
+    def test_streaming_line_empty_body(self):
+        stream = response.StreamingBody(
+            six.BytesIO(b''), content_length=0,
+        )
+        self.assert_lines(stream.iter_lines(), [])
+
 
 class FakeRawResponse(six.BytesIO):
     def stream(self, amt=1024, decode_content=None):
['tests/unit/test_response.py', 'botocore/response.py']