Add handling for an empty env arg when using the run method.

[hello-world-bfree]

Fixes #4

When using the run method, if the env arg is not set an AttributeError occurs.

The issue can be shown with the following snippet:

import os

env = None

for k, v in os.environ.items():
    env[k] = env.get(k, v)

Since there is no method get on NoneType the attribute error is raised.

[flarco]

Thank you, fixed with 628d477