pipenv update --outdated crash on empty requirements

[paradoxxxzero]

Not the most critical bug but:

$ pipenv install
(...)

$ pipenv update --outdated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