Redundant boundary if files/data is empty #862

If you try to send a post request with empty data and files — you will create a body with one --boundary-- string, which can break the server.

It's because httpx create MultipartStream for data={}, files={}:

And this MultipartStream doesn't have special behavior for empty data/files:

Reproducible example:

from httpx.content_streams import encode

stream = encode(data={}, files={}, boundary=b"+++")
print(b"".join([part for part in stream]))
# b'--+++--\r\n'

I see two ways to solve it:

1. Check files not only for None but also for emptiness in _content_streams.decode
2. Check the count of fields in MultipartStream.__init__ and doesn't write anything if fields are empty.

I already opened PR (#861) with a test example. If someone tells me which of these options is better, I will be happy to implement it 🙌