Pipenv uninstall --all returns error when there is no installed packages in virtuanlenv

[jxltom]

Issue description

In latest master branch, pipenv uninstall --all will returns error if the virtualenv is already fresh.

pipenv uninstall --all
Un-installing all packages from virtualenv...
Found 0 installed package(s), purging...
[pipenv.exceptions.UninstallError]:   File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/core.py", line 555, in invoke
[pipenv.exceptions.UninstallError]:       return callback(*args, **kwargs)
[pipenv.exceptions.UninstallError]:   File "/home/user/Documents/dev/pipenv/pipenv/vendor/click/decorators.py", line 17, in new_func
[pipenv.exceptions.UninstallError]:       return f(get_current_context(), *args, **kwargs)
[pipenv.exceptions.UninstallError]:   File "/home/user/Documents/dev/pipenv/pipenv/cli/command.py", line 294, in uninstall
[pipenv.exceptions.UninstallError]:       ctx=ctx
[pipenv.exceptions.UninstallError]:   File "/home/user/Documents/dev/pipenv/pipenv/core.py", line 2120, in do_uninstall
[pipenv.exceptions.UninstallError]:       do_purge(allow_global=system)
[pipenv.exceptions.UninstallError]:   File "/home/user/Documents/dev/pipenv/pipenv/core.py", line 1170, in do_purge
[pipenv.exceptions.UninstallError]:       raise exceptions.UninstallError(installed, command, c.out + c.err, c.return_code)
[pipenv.exceptions.UninstallError]: Attempted to run command: $ "/home/user/.local/share/virtualenvs/saleor-boilerplate-JGSSEPfX/bin/pip" uninstall  -y
[pipenv.exceptions.UninstallError]: You must give at least one requirement to uninstall (see "pip help uninstall")
ERROR: Failed to uninstall package(s) set([u'wheel', u'pip', u'setuptools'])...

It is better to just skip purging and show success information for this case, such as following result.

pipenv uninstall --all
Un-installing all packages from virtualenv...
Found 0 installed package(s), purging...
Environment now purged and fresh!