pipenv update --outdated crash on empty requirements
Not the most critical bug but:

```bash
$ pipenv install
(...)
```
```bash
$ pipenv update --outdated
Traceback (most recent call last):
  File "/usr/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/pipenv/cli.py", line 340, in update
    core.do_outdated()
  File "/usr/lib/python3.6/site-packages/pipenv/core.py", line 1705, in do_outdated
    packages.update(convert_deps_from_pip(result))
  File "/usr/lib/python3.6/site-packages/pipenv/utils.py", line 481, in convert_deps_from_pip
    req = get_requirement(dep)
  File "/usr/lib/python3.6/site-packages/pipenv/utils.py", line 128, in get_requirement
    req = [r for r in requirements.parse(dep)][0]
IndexError: list index out of range
```


diff --git a/pipenv/core.py b/pipenv/core.py
index 9886ede2..16e947f5 100644
--- a/pipenv/core.py
+++ b/pipenv/core.py
@@ -1701,6 +1701,7 @@ def do_py(system=False):
 def do_outdated():
     packages = {}
     results = delegator.run('{0} freeze'.format(which('pip'))).out.strip().split('\\n')
+    results = filter(bool, results)
     for result in results:
         packages.update(convert_deps_from_pip(result))
 
['pipenv/core.py']