Fix assumed hostname when using a 'file' URI scheme adapter

Commit 2255c34a65b5b1353004dc8d49cc397cd794ec15 in https://github.com/requests/requests/pull/4427 makes the assumption that all URLs have hostnames.  Or perhaps the assumption was that `urlparse()` returns an empty string for the `hostname` attribute rather than `None`.  Either way, the assumption blows up both for `is_ipv4_address(parsed.hostname)` and also for

```python
host_with_port = parsed.hostname
if parsed.port:
    host_with_port += ':{0}'.format(parsed.port)
```

when `parsed.hostname` is `None`.  Example circumstances of where this all blows up is when a `NO_PROXY` environment variable is set during use of something like conda's [adapter](https://github.com/conda/conda/blob/4.5.5/conda/gateways/connection/adapters/localfs.py) to handle `file:///` urls.  It results in a stack trace like

```
Traceback (most recent call last):
  File "/home/circleci/conda/tests/gateways/test_connection.py", line 62, in test_local_file_adapter_200
    r = session.get(test_url)
  File "/opt/conda/lib/python3.6/site-packages/requests/sessions.py", line 525, in get
    return self.request('GET', url, **kwargs)
  File "/opt/conda/lib/python3.6/site-packages/requests/sessions.py", line 503, in request
    prep.url, proxies, stream, verify, cert
  File "/opt/conda/lib/python3.6/site-packages/requests/sessions.py", line 676, in merge_environment_settings
    env_proxies = get_environ_proxies(url, no_proxy=no_proxy)
  File "/opt/conda/lib/python3.6/site-packages/requests/utils.py", line 760, in get_environ_proxies
    if should_bypass_proxies(url, no_proxy=no_proxy):
  File "/opt/conda/lib/python3.6/site-packages/requests/utils.py", line 716, in should_bypass_proxies
    if is_ipv4_address(parsed.hostname):
  File "/opt/conda/lib/python3.6/site-packages/requests/utils.py", line 640, in is_ipv4_address
    socket.inet_aton(string_ip)
TypeError: inet_aton() argument 1 must be str, not None
```

I *think* the correct solution here is to just return `True` in the `should_bypass_proxies()` function when there is no hostname in the URL.  Definitely know that sending `None` to `inet_aton(string_ip)` or trying to add `None` to a port string definitely isn't correct.

# [Codecov](https://codecov.io/gh/requests/requests/pull/4723?src=pr&el=h1) Report
> Merging [#4723](https://codecov.io/gh/requests/requests/pull/4723?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/dffd5d435e2d51958af1ccc9fd5e0f8f9df2fdfe?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4723/graphs/tree.svg?token=mZGswIiXLa&width=650&src=pr&height=150)](https://codecov.io/gh/requests/requests/pull/4723?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4723   +/-   ##
=======================================
  Coverage   66.79%   66.79%           
=======================================
  Files          15       15           
  Lines        1563     1563           
=======================================
  Hits         1044     1044           
  Misses        519      519
```



------

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Alright, I think this should work @kalefranz. Thanks for putting this together! I'd like @sigmavirus24 to give it a glance before we merge, if possible though.

Hi @kalefranz, would you mind rebasing this onto the master branch? Thanks!

Rebased.

Thanks @kalefranz! We’ll merge once the tests wrap.