TypeError: On `pipenv graph`
<details><summary>$ python -m pipenv.help output</summary>

Pipenv version: `'2018.05.18'`
Pipenv location: `'/usr/local/lib/python3.6/site-packages/pipenv'`
Python location: `'/usr/local/opt/python/bin/python3.6'`

Other Python installations in `PATH`:

  - `2.7`: `/usr/bin/python2.7`
  - `2.7`: `/usr/bin/python2.7`
  - `3.6`: `/usr/local/bin/python3.6m`
  - `3.6`: `/usr/local/bin/python3.6`

  - `2.7.10`: `/usr/bin/python`
  - `3.6.5`: `/usr/local/bin/python3`

PEP 508 Information:

```
{'implementation_name': 'cpython',
 'implementation_version': '3.6.5',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '17.5.0',
 'platform_system': 'Darwin',
 'platform_version': 'Darwin Kernel Version 17.5.0: Fri Apr 13 19:32:32 PDT '
                     '2018; root:xnu-4570.51.2~1/RELEASE_X86_64',
 'python_full_version': '3.6.5',
 'python_version': '3.6',
 'sys_platform': 'darwin'}
```

System environment variables:

  - `Apple_PubSub_Socket_Render`
  - `HOME`
  - `LC_CTYPE`
  - `LOGNAME`
  - `PATH`
  - `PWD`
  - `SECURITYSESSIONID`
  - `SHELL`
  - `SSH_AUTH_SOCK`
  - `TERM`
  - `TERM_PROGRAM`
  - `TERM_PROGRAM_VERSION`
  - `TERM_SESSION_ID`
  - `TMPDIR`
  - `TMUX`
  - `TMUX_PANE`
  - `USER`
  - `XPC_FLAGS`
  - `XPC_SERVICE_NAME`
  - `SHLVL`
  - `OLDPWD`
  - `PYTHONSTARTUP`
  - `PYTHONDONTWRITEBYTECODE`
  - `BETTER_EXCEPTIONS`
  - `EDITOR`
  - `LESS`
  - `HOMEBREW_AUTO_UPDATE`
  - `HOMEBREW_NO_AUTO_UPDATE`
  - `HOMEBREW_GITHUB_API_TOKEN`
  - `ZSH_MULTILINE`
  - `NMAPDIR`
  - `GOPATH`
  - `STUDIO_JDK`
  - `ANDROID_HOME`
  - `ANDROID_NDK`
  - `ONO_GAC_PREFIX`
  - `JAVA_HOME`
  - `JDK_HOME`
  - `JRE_HOME`
  - `CLASSPATH`
  - `MPLBACKEND`
  - `ITERMPLOT`
  - `ITERMPLOT_LINES`
  - `PS1`
  - `RPS1`
  - `_`
  - `__CF_USER_TEXT_ENCODING`
  - `PIP_PYTHON_PATH`

Pipenv–specific environment variables:


Debug–specific environment variables:

  - `PATH`: `/usr/local/opt/llvm/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/opt/sqlite/bin:/Users/gordio/Library/Python/3.6/bin:/usr/local/opt/go/libexec/bin:/Users/gordio/Development/Deps/jdk1.8.0_101.jdk/Contents/Home/bin:/Users/gordio/Development/Deps/android-sdk-macosx/platform-tools:/usr/local/opt/fzf/bin`
  - `SHELL`: `/usr/local/bin/zsh`
  - `EDITOR`: `/usr/local/bin/nvim`
  - `PWD`: `/Users/gordio/Development/myproj`


---------------------------

Contents of `Pipfile` ('/Users/gordio/Development/myproj/Pipfile'):

```toml

```

</details>

------------

##### Expected result

You know… :-)

##### Actual result

```
Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/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/local/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.6/site-packages/pipenv/vendor/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/pipenv/cli.py", line 850, in graph
    do_graph(bare=bare, json=json, reverse=reverse)
  File "/usr/local/lib/python3.6/site-packages/pipenv/core.py", line 2410, in do_graph
    python_path = which('python')
  File "/usr/local/lib/python3.6/site-packages/pipenv/core.py", line 121, in which
    location = project.virtualenv_location or os.environ.get('VIRTUAL_ENV')
  File "/usr/local/lib/python3.6/site-packages/pipenv/project.py", line 314, in virtualenv_location
    if not self.is_venv_in_project():
  File "/usr/local/lib/python3.6/site-packages/pipenv/project.py", line 219, in is_venv_in_project
    os.path.exists(os.path.join(self.project_directory, '.venv'))
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType
```

##### Steps to replicate

• As noob install `pip3 install pipenv` (python3 from home-brew)
• Run `pipenv graph`

diff --git a/pipenv/project.py b/pipenv/project.py
index 98dd4b22..986b7683 100644
--- a/pipenv/project.py
+++ b/pipenv/project.py
@@ -216,8 +216,8 @@ class Project(object):
         return bool(self.requirements_location)
 
     def is_venv_in_project(self):
-        return (
-            PIPENV_VENV_IN_PROJECT or
+        return PIPENV_VENV_IN_PROJECT or (
+            self.project_directory and
             os.path.exists(os.path.join(self.project_directory, '.venv'))
         )
 
['pipenv/project.py']